[GTPv1] Fix encoding of MM Context IE if NRSNRA bit not set

The len byte, describing the length of the field coming after it,
is always expected according to the struct definition.

lib/gtp/v1/types.c

@@ -504,6 +504,9 @@ int ogs_gtp1_build_mm_context(ogs_gtp1_tlv_mm_context_t *octet,
         CHECK_SPACE_ERR(2);
         *ptr++ = 1;
         *ptr++ = 0x01;
+    } else {
+        CHECK_SPACE_ERR(1);
+        *ptr++ = 0;
     }
 
     octet->len = (ptr - data);