[SGSN] SNDCP: Actually copy the payload data in the defrag queue entry

Prior to this patch our defragmented messages always consisted of 00-bytes
diff --git a/openbsc/src/gprs/gprs_sndcp.c b/openbsc/src/gprs/gprs_sndcp.c
index 46753c7..8ca9194 100644
--- a/openbsc/src/gprs/gprs_sndcp.c
+++ b/openbsc/src/gprs/gprs_sndcp.c
@@ -103,6 +103,8 @@
 	sne->defrag.seg_have |= (1 << seg_nr);
 	sne->defrag.tot_len += data_len;
 
+	memcpy(dqe->data, data, data_len);
+
 	return 0;
 }