modem-helpers: fix memory leak in mm_3gpp_parse_pdu_cmgl_response

This patch fixes a potential memory leak in
mm_3gpp_parse_pdu_cmgl_response() where the allocated MM3gppPduInfo may
be leaked when failing to parse the +CMGL response.
diff --git a/src/mm-modem-helpers.c b/src/mm-modem-helpers.c
index c999637..cf008d7 100644
--- a/src/mm-modem-helpers.c
+++ b/src/mm-modem-helpers.c
@@ -3626,6 +3626,7 @@
             list = g_list_append (list, info);
             g_match_info_next (match_info, &inner_error);
         } else {
+            mm_3gpp_pdu_info_free (info);
             inner_error = g_error_new (MM_CORE_ERROR,
                                        MM_CORE_ERROR_FAILED,
                                        "Error parsing +CMGL response: '%s'",