bssgp: check for IMSI being a null pointer, not just an empty string.
diff --git a/src/gb/gprs_bssgp.c b/src/gb/gprs_bssgp.c
index 4b8c730..8358819 100644
--- a/src/gb/gprs_bssgp.c
+++ b/src/gb/gprs_bssgp.c
@@ -811,7 +811,7 @@
 		}
 
 		/* IMSI */
-		if (strlen(dup->imsi)) {
+		if (dup->imsi && strlen(dup->imsi)) {
 			uint8_t mi[10];
 			int imsi_len = gsm48_generate_mid_from_imsi(mi, dup->imsi);
 			if (imsi_len > 2)