broadband-modem: clear error properly as it may be re-used

Avoids this kind of issues:

[mm-sms-list.c:334] mm_sms_list_take_part(): SMS part at 'me/0' is from a singlepart SMS
[mm-iface-modem-messaging.c:475] sms_added(): Added received SMS at '/org/freedesktop/ModemManager1/SMS/31'
[mm-broadband-modem.c:4791] sms_pdu_part_list_ready(): Error parsing PDU (1): PDU too short (2): 41 < 79

GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory.
This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
The overwriting error message was: PDU too short (3): 28 < (210 + 14), user data length: '203'

[mm-broadband-modem.c:4791] sms_pdu_part_list_ready(): Error parsing PDU (2): PDU too short (3): 28 < (210 + 14), user data length: '203'

src/mm-broadband-modem.c

@@ -4789,7 +4789,7 @@ sms_pdu_part_list_ready (MMBroadbandModem *self,
         } else {
             /* Don't treat the error as critical */
             mm_dbg ("Error parsing PDU (%d): %s", idx, error->message);
-            g_error_free (error);
+            g_clear_error (&error);
         }
     }