gprs_bssgp: Work around gcc-9 claiming "error=stringop-overflow"

gcc-9.2.1 issues the following errror when compiling with -Werror:

In function ‘tl16v_put’,
    inlined from ‘tvlv_put’ at ../../include/osmocom/gsm/tlv.h:156:9,
    inlined from ‘tvlv_put’ at ../../include/osmocom/gsm/tlv.h:147:24,
    inlined from ‘msgb_tvlv_put’ at ../../include/osmocom/gsm/tlv.h:223:9,
    inlined from ‘bssgp_tx_paging’ at gprs_bssgp.c:1250:2:
../../include/osmocom/gsm/tlv.h:131:2: error: ‘memcpy’ reading between 128 and 65535 bytes from a region of size 9 [-Werror=stringop-overflow=]
  131 |  memcpy(buf, val, len);
      |  ^~~~~~~~~~~~~~~~~~~~~

Unfortunately I've not been able to work around it with some nice GCC
	#pragma GCC diagnostic ignored "-Wstringop-overflow"

Change-Id: I22a0c399c6c00eaf87277002096a82844c9e198e
diff --git a/src/gb/gprs_bssgp.c b/src/gb/gprs_bssgp.c
index 896f1c5..38794c2 100644
--- a/src/gb/gprs_bssgp.c
+++ b/src/gb/gprs_bssgp.c
@@ -1178,6 +1178,7 @@
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
 		int imsi_len = gsm48_generate_mid_from_imsi(mi, dup->imsi);
+		OSMO_ASSERT(imsi_len <= GSM48_MID_MAX_SIZE);
 		if (imsi_len > 2)
 			msgb_tvlv_push(msg, BSSGP_IE_IMSI,
 				imsi_len-2, mi+2);
@@ -1247,6 +1248,7 @@
  * mi[131], which is wrong */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
+	OSMO_ASSERT(imsi_len <= GSM48_MID_MAX_SIZE);
 	msgb_tvlv_put(msg, BSSGP_IE_IMSI, imsi_len-2, mi+2);
 #pragma GCC diagnostic pop
 	/* DRX Parameters */