Fix memory leaks with PB_ENABLE_MALLOC and certain submessage type combinations.

There was a memory leak when:

1) A statically allocated submessage or
2) an extension field submessage

contained

A) a pointer-type field or
B) a submessage that further contained a pointer-type field.

This was because pb_release() didn't recurse into non-pointer fields.

Update issue 138
Status: FixedInGit