[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.