fix luop crash: use buffer for APN that remains valid

In osmo_gsup_configure_wildcard_apn(), do not compose APN into a local buffer
that becomes invalid as soon as the function exits. Instead, use a caller
provided buf.

Fixes OS#3231 crash:

  ==20030==ERROR: AddressSanitizer: stack-buffer-underflow on address 0x7fffffffd9c0 at pc 0x7ffff6e9b6c2 bp 0x7fffffffd900 sp 0x7fffffffd0b0
  READ of size 2 at 0x7fffffffd9c0 thread T0
      #0 0x7ffff6e9b6c1  (/usr/lib/x86_64-linux-gnu/libasan.so.4+0x766c1)
      #1 0x7ffff6314419 in tlv_put ../../../../src/libosmocore/include/osmocom/gsm/tlv.h:107
      #2 0x7ffff6314419 in msgb_tlv_put ../../../../src/libosmocore/include/osmocom/gsm/tlv.h:299
      #3 0x7ffff6314419 in encode_pdp_info ../../../../src/libosmocore/src/gsm/gsup.c:419
      #4 0x7ffff6314419 in osmo_gsup_encode ../../../../src/libosmocore/src/gsm/gsup.c:535
      #5 0x555555580016 in _luop_tx_gsup ../../../src/osmo-hlr/src/luop.c:54
      #6 0x5555555809d8 in lu_op_tx_insert_subscr_data ../../../src/osmo-hlr/src/luop.c:264
      #7 0x55555558b356 in rx_upd_loc_req ../../../src/osmo-hlr/src/hlr.c:306
      #8 0x55555558b356 in read_cb ../../../src/osmo-hlr/src/hlr.c:365
      #9 0x555555586671 in osmo_gsup_server_read_cb ../../../src/osmo-hlr/src/gsup_server.c:105
      #10 0x7ffff5b35911 in ipa_server_conn_read ../../../src/libosmo-abis/src/input/ipa.c:356
      #11 0x7ffff5b35911 in ipa_server_conn_cb ../../../src/libosmo-abis/src/input/ipa.c:387
      #12 0x7ffff5e5541f in osmo_fd_disp_fds ../../../src/libosmocore/src/select.c:216
      #13 0x7ffff5e5541f in osmo_select_main ../../../src/libosmocore/src/select.c:256
      #14 0x5555555791b6 in main ../../../src/osmo-hlr/src/hlr.c:600
      #15 0x7ffff4707a86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21a86)
      #16 0x555555579679 in _start (/usr/local/bin/osmo-hlr+0x25679)

  Address 0x7fffffffd9c0 is located in stack of thread T0 at offset 16 in frame
      #0 0x7ffff63131ff in osmo_gsup_encode ../../../../src/libosmocore/src/gsm/gsup.c:481

    This frame has 1 object(s):
      [32, 64) 'bcd_buf' <== Memory access at offset 16 underflows this variable

Related: OS#3231
Change-Id: I83d9ef2868bbb01e3f1ddb7920fe735aca172b15
diff --git a/src/luop.c b/src/luop.c
index 7150b64..54c3884 100644
--- a/src/luop.c
+++ b/src/luop.c
@@ -27,6 +27,7 @@
 #include <osmocom/core/logging.h>
 #include <osmocom/gsm/gsm48_ie.h>
 #include <osmocom/gsm/gsup.h>
+#include <osmocom/gsm/apn.h>
 
 #include "gsup_server.h"
 #include "gsup_router.h"
@@ -233,6 +234,7 @@
 {
 	struct osmo_gsup_message gsup;
 	uint8_t msisdn_enc[43]; /* TODO use constant; TS 24.008 10.5.4.7 */
+	uint8_t apn[APN_MAXLEN];
 	int l;
 
 	OSMO_ASSERT(luop->state == LU_S_LU_RECEIVED ||
@@ -257,7 +259,7 @@
 	if (luop->is_ps) {
 		/* FIXME: PDP infos - use more fine-grained access control
 		   instead of wildcard APN */
-		osmo_gsup_configure_wildcard_apn(&gsup);
+		osmo_gsup_configure_wildcard_apn(&gsup, apn, sizeof(apn));
 	}
 
 	/* Send ISD to new VLR/SGSN */