Reset minimum alignment when reusing FlatBufferBuilder

Previously, if you were re-using instances of FlatBufferBuilder
and an earlier buffer would use a 64bit item where later ones
do not, you could be wasting space.

Change-Id: Ic8090a38f97ce73194e991ba72bcfae74a7ace9f
Tested: on Linux.

include/flatbuffers/flatbuffers.h

@@ -454,6 +454,7 @@ class FlatBufferBuilder FLATBUFFERS_FINAL_CLASS {
     buf_.clear();
     offsetbuf_.clear();
     vtables_.clear();
+    minalign_ = 1;
   }
 
   // The current size of the serialized buffer, counting from the end.