Fix a fallthrough bug in TranslatorX64::smash

If smash is told to generate a forward jump to a target less
than 7 bytes away, it nops out the intermediate region.  It's
currently impossible for this to cause any issues (as far as I know),
but technically there can be other instructions in there.  I hit this
by randomizing the layout of blocks in codegen---if you place a block
consisting of only a short backward jump immediately after the block
containing the main trace exit, and then a fallthrough-ish translation
occurs, smash will nop out the short-jump block.

hphp/runtime/vm/translator/translator-x64.cpp

@@ -1542,7 +1542,7 @@ TranslatorX64::smash(X64Assembler &a, TCA src, TCA dest, bool isCall) {
    */
   CodeCursor cg(a, src);
   assert(isSmashable(a.code.frontier, kJmpLen));
-  if (dest > src && dest - src <= 7) {
+  if (dest > src && dest - src <= kJmpLen) {
     assert(!isCall);
     a.    emitNop(dest - src);
   } else if (!isCall) {