Fix a bug with null constants

We end up with uninit nulls in the repo for unit constants.
In RepoAuthoritative mode, we will use m_type == KindOfUninit during
constant lookup to decide whether to cast TypedValue::m_data.pref to a
ConstantInfo*, and then call a function pointer that it contains.  (We
should probably also clean that up to not cast RefData*'s to
ConstantInfo*'s, but I left that out of this diff.)

hphp/compiler/analysis/emitter.cpp

@@ -1686,6 +1686,9 @@ void EmitterVisitor::visit(FileScopePtr file) {
                 if (func->isSimpleDefine(&name, &tv)) {
                   UnitMergeKind k = func->isDefineWithoutImpl(ar) ?
                     UnitMergeKindPersistentDefine : UnitMergeKindDefine;
+                  if (tv.m_type == KindOfUninit) {
+                    tv.m_type = KindOfNull;
+                  }
                   m_ue.pushMergeableDef(k, name, tv);
                   visit(s);
                   continue;

hphp/runtime/vm/unit.cpp

@@ -1273,6 +1273,7 @@ void Unit::mergeImpl(void* tcbase, UnitMergeInfo* mi) {
           Stats::inc(Stats::UnitMerge_mergeable_define);
           StringData* name = (StringData*)((char*)obj - (int)k);
           auto* v = (TypedValueAux*)mi->mergeableData(ix + 1);
+          assert(v->m_type != KindOfUninit);
           mergeCns(getDataRef<TypedValue>(tcbase, v->cacheHandle()), v, name);
           ix += 1 + sizeof(*v) / sizeof(void*);
           obj = mi->mergeableObj(ix);

hphp/test/quick/null_constant.php

@@ -0,0 +1,2 @@
+<?php
+define('FOO', null);