Made the memcmp address sanitizer clean.

Added extra check to ensure memcmp gets called with a size that is
guaranteed within range of the buffer. This wasn't a real problem,
but stops address sanitizer from complaining.

See:
https://github.com/google/flatbuffers/issues/88
https://github.com/joker-eph/flatbuffers/commit/517506b4e105814e0823353c3d6561085c26a1ab#commitcomment-8265231

Change-Id: I7de24da2d36d973e154f92eeb3e093070886037f
Tested: on Linux

include/flatbuffers/flatbuffers.h

@@ -540,7 +540,9 @@ class FlatBufferBuilder {
     // See if we already have generated a vtable with this exact same
     // layout before. If so, make it point to the old one, remove this one.
     for (auto it = vtables_.begin(); it != vtables_.end(); ++it) {
-      if (memcmp(buf_.data_at(*it), vt1, vt1_size)) continue;
+      auto vt2 = reinterpret_cast<voffset_t *>(buf_.data_at(*it));
+      auto vt2_size = *vt2;
+      if (vt1_size != vt2_size || memcmp(vt2, vt1, vt1_size)) continue;
       vt_use = *it;
       buf_.pop(GetSize() - vtableoffsetloc);
       break;