[PGO] Don't call calloc(0, sizeof(ValueProfNode *))

A malloc implementation may return a pointer to some allocated space.  It is
undefined for libclang_rt.profile- to access the object - which actually happens
in instrumentTargetValueImpl, where ValueCounters[CounterIndex] may access a
ValueProfNode (from another allocated object) and crashes when the code accesses
the object referenced by CurVNode->Next.

compiler-rt/lib/profile/InstrProfilingValue.c

@@ -93,6 +93,8 @@ static int allocateValueProfileCounters(__llvm_profile_data *Data) {
   for (VKI = IPVK_First; VKI <= IPVK_Last; ++VKI)
     NumVSites += Data->NumValueSites[VKI];
 
+  if (NumVSites == 0)
+    return 0;
   ValueProfNode **Mem =
       (ValueProfNode **)calloc(NumVSites, sizeof(ValueProfNode *));
   if (!Mem)