[slam] Avoid NULL pointer dereference in slam_pull_value()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/slam.c

@@ -400,13 +400,17 @@ static int slam_pull_value ( struct slam_request *slam,
 		return -EINVAL;
 	}
 
-	/* Read value */
+	/* Strip value */
 	iob_pull ( iobuf, len );
+
+	/* Read value, if applicable */
+	if ( value ) {
 		*value = ( *data & 0x1f );
 		while ( --len ) {
 			*value <<= 8;
 			*value |= *(++data);
 		}
+	}
 
 	return 0;
 }