[Fix fuzzer error] Memory address points to zero page. (#32894)

Found memory access error in frame_fuzzer_test. Located the root cause
in ExecCtx::Get(), where ExecCtx needs to be initialized before using
HPackParser:ParseInput().


Error logs:
MemorySanitizer:DEADLYSIGNAL
==2812845==ERROR: MemorySanitizer: SEGV on unknown address
0x000000000030 (pc 0x55869275574e bp 0x7fffd7d9fb50 sp 0x7fffd7d9fb20
T2812845)
==2812845==The signal is caused by a READ memory access.
==2812845==Hint: address points to the zero page.
#0 0x55869275574e in starting_cpu
[third_party/grpc/src/core/lib/iomgr/exec_ctx.h:129](https://cs.corp.google.com/piper///depot/google3/third_party/grpc/src/core/lib/iomgr/exec_ctx.h?l=129&ws=ladynana/2900&snapshot=42):9
#1 0x55869275574e in
grpc_core::PerCpu<grpc_core::GlobalStatsCollector::Data>::this_cpu()
[third_party/grpc/src/core/lib/gprpp/per_cpu.h:38](https://cs.corp.google.com/piper///depot/google3/third_party/grpc/src/core/lib/gprpp/per_cpu.h?l=38&ws=ladynana/2900&snapshot=42):48
#2 0x558692753cda in IncrementHttp2MetadataSize
[third_party/grpc/src/core/lib/debug/stats_data.h:265](https://cs.corp.google.com/piper///depot/google3/third_party/grpc/src/core/lib/debug/stats_data.h?l=265&ws=ladynana/2900&snapshot=42):11
#3 0x558692753cda in
grpc_core::HPackParser::ParseInput(grpc_core::HPackParser::Input, bool)
[third_party/grpc/src/core/ext/transport/chttp2/transport/hpack_parser.cc:933](https://cs.corp.google.com/piper///depot/google3/third_party/grpc/src/core/ext/transport/chttp2/transport/hpack_parser.cc?l=933&ws=ladynana/2900&snapshot=42):20


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test/core/transport/chaotic_good/BUILD

@@ -61,6 +61,7 @@ grpc_fuzzer(
     language = "C++",
     tags = ["no_windows"],
     deps = [
+        "//:exec_ctx",
         "//:gpr",
         "//:hpack_encoder",
         "//:hpack_parser",

test/core/transport/chaotic_good/frame_fuzzer.cc

@@ -27,6 +27,7 @@
 #include "src/core/ext/transport/chttp2/transport/hpack_encoder.h"
 #include "src/core/ext/transport/chttp2/transport/hpack_parser.h"
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
 #include "src/core/lib/resource_quota/arena.h"
 #include "src/core/lib/resource_quota/memory_quota.h"
 #include "src/core/lib/resource_quota/resource_quota.h"
@@ -61,6 +62,7 @@ template <typename T>
 void FinishParseAndChecks(const FrameHeader& header, const uint8_t* data,
                           size_t size) {
   T parsed;
+  ExecCtx exec_ctx;  // Initialized to get this_cpu() info in global_stat().
   HPackParser hpack_parser;
   SliceBuffer serialized;
   serialized.Append(Slice::FromCopiedBuffer(data, size));