res_fax_spandsp: fix t38 transmission bug caused by not returning success

This patch fixes the problem, but the issue includes a test which is still
being considered for the automated test suite.

(issue ASTERISK-20919)
Reported by: NITESH BANSAL
Patches:
	patch_ast_fax_spandsp.patch uploaded by NITESH BANSAL (license 6418)
........

Merged revisions 379949 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379950 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_fax_spandsp.c

@@ -227,6 +227,7 @@ static int t38_tx_packet_handler(t38_core_state_t *t38_core_state, void *data, c
 	} else {
 		/* no need to lock, this all runs in the same thread */
 		AST_LIST_INSERT_TAIL(&p->read_frames, f, frame_list);
+		res = 0;
 	}
 
 	return res;