gsm: handle case of entirely empty SPN correctly

BUG=none
TEST=Insert a SIM with a present but empty (all 0xFF) SPN and check
the system log for a (lack of) assertion errors from mm_charset_gsm_unpacked_to_utf8().

Change-Id: I1250494b9757c9bfdce56402a4471c598f41223f
Reviewed-on: http://gerrit.chromium.org/gerrit/3139
Reviewed-by: Eric Shienbrood <ers@chromium.org>
Tested-by: Nathan J. Williams <njw@chromium.org>

src/mm-generic-gsm.c

@@ -2002,7 +2002,7 @@ get_spn_done (MMAtSerialPort *port,
         }
 
         /* Remove the FF filler at the end */
-        while (bin[buflen - 1] == (char)0xff)
+        while (buflen > 1 && bin[buflen - 1] == (char)0xff)
             buflen--;
 
         /* First byte is metadata; remainder is GSM-7 unpacked into octets; convert to UTF8 */