Fixed some small bugs and performance problems in string copying.

Before this CL, with alias=false:

------------------------------------------------------------------------------
Benchmark                                       Time           CPU Iterations
------------------------------------------------------------------------------
BM_Parse_Upb_FileDesc_WithInitialBlock       3715 ns       3715 ns     188916   1.88206GB/s

 Performance counter stats for 'bazel-bin/benchmarks/benchmark --benchmark_filter=BM_Parse_Upb_FileDesc_WithInitialBlock':

          1,122.92 msec task-clock                #    0.979 CPUs utilized
                 3      context-switches          #    0.003 K/sec
                 0      cpu-migrations            #    0.000 K/sec
               196      page-faults               #    0.175 K/sec
     4,144,746,717      cycles                    #    3.691 GHz
    15,351,966,804      instructions              #    3.70  insn per cycle
     2,590,281,905      branches                  # 2306.728 M/sec
         2,996,157      branch-misses             #    0.12% of all branches

       1.146615328 seconds time elapsed

       1.115578000 seconds user
       0.008025000 seconds sys

After this CL:

------------------------------------------------------------------------------
Benchmark                                       Time           CPU Iterations
------------------------------------------------------------------------------
BM_Parse_Upb_FileDesc_WithInitialBlock       3554 ns       3554 ns     197527    1.9674GB/s

 Performance counter stats for 'bazel-bin/benchmarks/benchmark --benchmark_filter=BM_Parse_Upb_FileDesc_WithInitialBlock':

          1,105.34 msec task-clock                #    0.982 CPUs utilized
                 3      context-switches          #    0.003 K/sec
                 0      cpu-migrations            #    0.000 K/sec
               197      page-faults               #    0.178 K/sec
     4,077,736,892      cycles                    #    3.689 GHz
    15,442,709,352      instructions              #    3.79  insn per cycle
     2,435,131,301      branches                  # 2203.068 M/sec
         2,643,775      branch-misses             #    0.11% of all branches

       1.125393845 seconds time elapsed

       1.097770000 seconds user
       0.008012000 seconds sys

upb/decode_fast.c

@@ -472,11 +472,11 @@ static const char *fastdecode_longstring(struct upb_decstate *d,
 UPB_FORCEINLINE
 static void fastdecode_docopy(upb_decstate *d, const char *ptr, uint32_t size,
                               int copy, char *data, upb_strview *dst) {
+  d->arena.head.ptr += copy;
   UPB_UNPOISON_MEMORY_REGION(data, copy);
   memcpy(data, ptr, copy);
   UPB_POISON_MEMORY_REGION(data + size, copy - size);
   dst->data = data;
-  d->arena.head.ptr += copy;
 }
 
 UPB_FORCEINLINE
@@ -504,35 +504,30 @@ again:
   ptr += tagbytes + 1;
   dst->size = size;
 
-  if (UPB_UNLIKELY(size == 0)) {
-    goto done;
-  } else if (UPB_UNLIKELY(size > 127)) {
-    goto longstr;
-  }
-
   buf = d->arena.head.ptr;
   arena_has = _upb_arenahas(&d->arena);
   common_has = UPB_MIN(arena_has, (d->end - ptr) + 16);
 
   if (UPB_LIKELY(size <= 15 - tagbytes)) {
     if (arena_has < 16) goto longstr;
+    d->arena.head.ptr += 16;
     memcpy(buf, ptr - tagbytes - 1, 16);
     dst->data = buf + tagbytes + 1;
-    d->arena.head.ptr += 16;
   } else if (UPB_LIKELY(size <= 32)) {
     if (UPB_UNLIKELY(common_has < 32)) goto longstr;
     fastdecode_docopy(d, ptr, size, 32, buf, dst);
-  } else if (UPB_LIKELY(size <= 64 && common_has >= 64)) {
+  } else if (UPB_LIKELY(size <= 64)) {
     if (UPB_UNLIKELY(common_has < 64)) goto longstr;
     fastdecode_docopy(d, ptr, size, 64, buf, dst);
-  } else {
+  } else if (UPB_LIKELY(size <= 128)) {
     if (UPB_UNLIKELY(common_has < 128)) goto longstr;
     fastdecode_docopy(d, ptr, size, 128, buf, dst);
+  } else {
+    goto longstr;
   }
 
   ptr += size;
 
-done:
   if (card == CARD_r) {
     fastdecode_nextret ret = fastdecode_nextrepeated(
         d, dst, &ptr, &farr, data, tagbytes, sizeof(upb_strview));