Remove bogus assert in bytecode emitter

I hit an array on the debug build when I tried to run the following
program:

  <?php
  function f() {
    for (;;) {}
  }
  f();

This diff removes the offending assert since it is technically possible to
have m_maxStackDepth be equal to 0.

hphp/runtime/vm/func.cpp

@@ -964,7 +964,6 @@ Func* FuncEmitter::create(Unit& unit, PreClass* preClass /* = NULL */) const {
   f->shared()->m_numLocals = m_numLocals;
   f->shared()->m_numIterators = m_numIterators;
   f->m_maxStackCells = m_maxStackCells;
-  assert(m_maxStackCells > 0 && "You probably didn't set m_maxStackCells");
   f->shared()->m_staticVars = m_staticVars;
   f->shared()->m_ehtab = m_ehtab;
   f->shared()->m_fpitab = m_fpitab;

hphp/test/quick/zero_eval_stack_depth.php

@@ -0,0 +1,5 @@
+<?php
+function f() {
+  for (;;) {}
+}
+echo "Done\n";

hphp/test/quick/zero_eval_stack_depth.php.expect

@@ -0,0 +1 @@
+Done