libbsc/bsc_vty.c: prevent uninitialized access

If an out of range 'Last Valid Block' value for 'smscb-command'
is passed, a 'last_block' of the 'rsl_ie_cb_cmd_type' struct
could be uninitialized. Let's prevent this.

Found using Clang Static Analyzer.

Change-Id: I57635f2f482ff476ab697b1b9e872ce90aafb999

src/libbsc/bsc_vty.c

@@ -4258,6 +4258,9 @@ DEFUN(smscb_cmd, smscb_cmd_cmd,
 	case 4:
 		cb_cmd.last_block = RSL_CB_CMD_LASTBLOCK_4;
 		break;
+	default:
+		vty_out(vty, "Error parsing LASTBLOCK%s", VTY_NEWLINE);
+		return CMD_WARNING;
 	}
 
 	rsl_sms_cb_command(bts, RSL_CHAN_SDCCH4_ACCH, cb_cmd, buf, rc);