Fix memory leak with duplicated fields and PB_ENABLE_MALLOC.

If a required or optional field appeared twice in the message data,
pb_decode will overwrite the old data with new one. That is fine, but
with submessage fields, it didn't release the allocated subfields before
overwriting.

This bug can manifest if all of the following conditions are true:

1. There is a message with a "optional" or "required" submessage field
   that has type:FT_POINTER.

2. The submessage contains atleast one field with type:FT_POINTER.

3. The message data to be decoded has the submessage field twice in it.