gbproxy: Add gbprox_clear_patch_filter() (Coverity)

Add a separate function to clear the IMSI filter to be used instead of
gbprox_set_patch_filter(cfg, NULL, ...). Albeit it fixes a Coverity
issue (Unchecked return value), it is a false positive, since the
return value is always 0 in these cases. Nevertheless it is more
obvious what happens when an explicit clear function is called. Using
NULL as filter argument of gbprox_set_patch_filter still clears the
filter.

Fixes: Coverity CID 1231255
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index ec73ae6..9574a45 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -193,7 +193,7 @@
 		talloc_free(g_cfg->core_apn);
 		g_cfg->core_apn = NULL;
 		g_cfg->core_apn_size = 0;
-		gbprox_set_patch_filter(g_cfg, NULL, NULL);
+		gbprox_clear_patch_filter(g_cfg);
 		return CMD_SUCCESS;
 	}
 
@@ -206,7 +206,7 @@
 	}
 
 	if (!filter) {
-		gbprox_set_patch_filter(g_cfg, NULL, NULL);
+		gbprox_clear_patch_filter(g_cfg);
 	} else if (gbprox_set_patch_filter(g_cfg, filter, &err_msg) != 0) {
 		vty_out(vty, "Match expression invalid: %s%s",
 			err_msg, VTY_NEWLINE);