Fix alignment issue in gpr_murmur_hash3

This function cast a void* to a uint32_t*. This is invalid, since a uint32_t* must be 32-bit-aligned, while the input key clearly isn't. Even though the function later uses memcpy to access the memory, by that point the compiler is allowed to assume that the pointer is aligned, and so it can output code that does an unaligned memory access.

In practice, this resulted in a crash on some devices when this code is compiled with optimizations for 32-bit ARM with the Android NDK r14.

src/core/lib/support/murmur_hash.cc

@@ -30,22 +30,19 @@
   (h) ^= (h) >> 16;
 
 uint32_t gpr_murmur_hash3(const void* key, size_t len, uint32_t seed) {
-  const uint8_t* data = (const uint8_t*)key;
-  const size_t nblocks = len / 4;
-  int i;
-
   uint32_t h1 = seed;
   uint32_t k1;
 
   const uint32_t c1 = 0xcc9e2d51;
   const uint32_t c2 = 0x1b873593;
 
-  const uint32_t* blocks = ((const uint32_t*)key) + nblocks;
-  const uint8_t* tail = (const uint8_t*)(data + nblocks * 4);
+  const uint8_t* keyptr = (const uint8_t*)key;
+  const size_t bsize = sizeof(k1);
+  const size_t nblocks = len / bsize;
 
   /* body */
-  for (i = -(int)nblocks; i; i++) {
-    memcpy(&k1, blocks + i, sizeof(uint32_t));
+  for (size_t i = 0; i < nblocks; i++, keyptr += bsize) {
+    memcpy(&k1, keyptr, bsize);
 
     k1 *= c1;
     k1 = ROTL32(k1, 15);
@@ -61,13 +58,13 @@ uint32_t gpr_murmur_hash3(const void* key, size_t len, uint32_t seed) {
   /* tail */
   switch (len & 3) {
     case 3:
-      k1 ^= ((uint32_t)tail[2]) << 16;
+      k1 ^= ((uint32_t)keyptr[2]) << 16;
     /* fallthrough */
     case 2:
-      k1 ^= ((uint32_t)tail[1]) << 8;
+      k1 ^= ((uint32_t)keyptr[1]) << 8;
     /* fallthrough */
     case 1:
-      k1 ^= tail[0];
+      k1 ^= keyptr[0];
       k1 *= c1;
       k1 = ROTL32(k1, 15);
       k1 *= c2;