mgcp_client: simpler error handling

add_sdp(), add_lco():

- do not msgb_free() within these functions. Just return error, and move
  the msgb_free() to the caller.

- when failing to write to the msgb, directly return error.

Change-Id: I904d56f56053952c2ebbbe5dca744fafa32b333e
diff --git a/tests/mgcp_client/mgcp_client_test.err b/tests/mgcp_client/mgcp_client_test.err
index d666cba..0bf6d8f 100644
--- a/tests/mgcp_client/mgcp_client_test.err
+++ b/tests/mgcp_client/mgcp_client_test.err
@@ -1,5 +1,5 @@
 DLMGCP MGW(mgw) MGCP client: using endpoint domain '@mgw'
-DLMGCP MGW(mgw) Message buffer to small, can not generate MGCP message (SDP)
+DLMGCP MGW(mgw) Message buffer too small, can not generate MGCP message (SDP)
 DLMGCP MGW(mgw) Failed to add SDP, can not generate MGCP message
 
 test_mgcp_client_cancel():