rspro_server.c: Fix memory leak in processing inbound RSPRO

We need to unconditionally free the msgb that gets handed into
sock_read_cb() by the ipa server.

Closes: OS#4096
Change-Id: I12b4a22854eea79c9e9c3e565dd4803e4dc0ed5a
diff --git a/src/server/rspro_server.c b/src/server/rspro_server.c
index 7cf98ad..c753f16 100644
--- a/src/server/rspro_server.c
+++ b/src/server/rspro_server.c
@@ -613,6 +613,7 @@
 	default:
 		goto invalid;
 	}
+	msgb_free(msg);
 	return rc;
 
 invalid: