gsup: Copy the hlr-Number and use it during purge

Copy the hlr-Number into the sgsn_data and use it during
the purgeMS. There is no unit test that looks at the data
we send so I manually verified this by looking at the output.
Below is the output of the test that purges the subscriber.

<000f> gprs_subscriber.c:170 SUBSCR(123456789012345) Sending GSUP, will send: 0c 01 08 21 43 65 87 09 21 43 f5 09 07 91 83 61 26 31 23 f3
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index 879dfe3..a06c153 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -1155,6 +1155,8 @@
 			0x12, 0x09, 0x04, 't', 'e', 's', 't', 0x03, 'a', 'p', 'n',
 		0x08, 0x07, /* MSISDN 49166213323 encoded */
 			0x91, 0x94, 0x61, 0x26, 0x31, 0x23, 0xF3,
+		0x09, 0x07, /* MSISDN 38166213323 encoded */
+			0x91, 0x83, 0x61, 0x26, 0x31, 0x23, 0xF3,
 	};
 
 	OSMO_ASSERT(!mmctx || mmctx->subscr);