Fix pointer underrun when allocating large vectors

Change-Id: Ia69fc3098468eb64420215dc1068342ccbbb1ede

include/flatbuffers/flatbuffers.h

@@ -367,7 +367,7 @@ class vector_downward {
   }
 
   uint8_t *make_space(size_t len) {
-    if (buf_ > cur_ - len) {
+    if (len > static_cast<size_t>(cur_ - buf_)) {
       auto old_size = size();
       reserved_ += std::max(len, growth_policy(reserved_));
       auto new_buf = allocator_.allocate(reserved_);