Avoid mixed signedness compare in gzwrite.c.
diff --git a/gzwrite.c b/gzwrite.c
index e7864cb..9348473 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -380,7 +380,7 @@
     z_streamp strm;
 
     strm = &(state->strm);
-    if ((strm->next_in + strm->avail_in) - state->in <= state->size)
+    if (strm->next_in + strm->avail_in <= state->in + state->size)
         return 0;
     (void)gz_comp(state, Z_NO_FLUSH);
     if (strm->avail_in == 0) {