Fix some stack overflow bugs

If re-entry triggers stack overflow, there is no FCall or FCallArray,
so the code to find the pc in REQ_STACK_OVERFLOW would fail. If it
managed to get through there, it would then try to DecRef the contents
of an ActRec, sometimes causing it to DecRef a Func.

In addition, if a "leaf" function resulted in re-entry we didn't
necessarily check the stack.

Differential Revision: D936843

hphp/runtime/vm/bytecode.cpp

@@ -1655,7 +1655,8 @@ void VMExecutionContext::invokeFunc(TypedValue* retval,
   }
   Cell* savedSP = m_stack.top();
 
-  if (f->numParams() > kStackCheckReenterPadding - kNumActRecCells) {
+  if (f->attrs() & AttrPhpLeafFn ||
+      f->numParams() > kStackCheckReenterPadding - kNumActRecCells) {
     checkStack(m_stack, f);
   }
 
@@ -1797,7 +1798,8 @@ void VMExecutionContext::invokeFuncFew(TypedValue* retval,
     thiz->incRefCount();
   }
   Cell* savedSP = m_stack.top();
-  if (argc > kStackCheckReenterPadding - kNumActRecCells) {
+  if (f->attrs() & AttrPhpLeafFn ||
+      argc > kStackCheckReenterPadding - kNumActRecCells) {
     checkStack(m_stack, f);
   }
   ActRec* ar = m_stack.allocA();