gtp_error_ind_conf(): fix: guard against an unknown GTP version

This change fixes the following compiler warnings (found by Clang):

  gtp.c:2747:13: warning: variable 'pdp' is used uninitialized
			  whenever 'if' condition is false
			  [-Wsometimes-uninitialized]
		 } else if (version == 1) {

  gtp.c:2781:14: note: uninitialized use occurs here
  		 OSMO_ASSERT(pdp);
			     ^^^

Shall not happen in general, but let's make Clang happy.

Change-Id: Id471b22afd4c45435589a4edda0a804e66be3a7a
diff --git a/gtp/gtp.c b/gtp/gtp.c
index 7cb82b6..3979165 100644
--- a/gtp/gtp.c
+++ b/gtp/gtp.c
@@ -2743,6 +2743,9 @@
 			GTP_LOGPKG(LOGL_ERROR, peer, pack, len, "Unknown PDP context\n");
 			return EOF;
 		}
+	} else {
+		LOGP(DLGTP, LOGL_ERROR, "Unknown version: %d\n", version);
+		return EOF;
 	}
 
 	GTP_LOGPKG(LOGL_ERROR, peer, pack, len,