MPI_Unpack: fix error code when insize <= 0

this fixes a regression from open-mpi/ompi@f2e33c725f

ompi/mpi/c/unpack.c

@@ -101,6 +101,8 @@ int MPI_Unpack(const void *inbuf, int insize, int *position,
         rc = opal_convertor_unpack( &local_convertor, &outvec, &iov_count, &size );
         *position += size;
         OBJ_DESTRUCT( &local_convertor );
+    } else {
+        rc = 1;
     }
 
     OPAL_CR_EXIT_LIBRARY();