smpp: Fix compilation warning
I already stumbled into 2 compilation environments which had Werror
enabled for -Wmaybe-uninitialized and the build failed, so let's
workaround this warning.
| smpp_openbsc.c: In function 'handle_smpp_submit':
| smpp_openbsc.c:216:9: error: 'sms_msg_len' may be used uninitialized in this function [-Werror=maybe-uninitialized]
| memcpy(sms->user_data, sms_msg, sms_msg_len);
| ^
| smpp_openbsc.c:100:15: note: 'sms_msg_len' was declared here
| unsigned int sms_msg_len;
| ^
| cc1: some warnings being treated as errors
Change-Id: I0901ddadb5f72e1585cb1797ac22c8ab95e83146