[tsan] Check for nullptr on user_alloc_usable_size_fast

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Summary

MBlock could be null (in the context of a call from RunFreeHooks)
if a static object was runtime initialized before tsan finished
initializing and that object later did a free().

While having the check isn't strictly required by
__sanitizer_get_allocated_size_fast's contract, a user's static object
would expect ptrs returned from malloc to be valid inputs.

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compiler-rt/lib/tsan/rtl/tsan_mman.cpp

if (!b)
     return 0;  // Not a valid pointer.
   if (b->siz == 0)
     return 1;  // Zero-sized allocations are actually 1 byte.
   return b->siz;
 }
 
 uptr user_alloc_usable_size_fast(const void *p) {
   MBlock *b = ctx->metamap.GetBlock((uptr)p);
+  // Static objects may have malloc'd before tsan completes
+  // initialization, and may believe returned ptrs to be valid.
+  if (!b)
+    return 0;  // Not a valid pointer.
   if (b->siz == 0)
     return 1;  // Zero-sized allocations are actually 1 byte.
   return b->siz;
 }
 
 void invoke_malloc_hook(void *ptr, uptr size) {
   ThreadState *thr = cur_thread();
   if (ctx == 0 || !ctx->initialized || thr->ignore_interceptors)