Fix buf error on interrupt.

Author: samuel-williams-shopify

ext/zlib/zlib.c

@@ -1173,9 +1173,11 @@ zstream_run_try(VALUE value_arg)
        goto loop;
     }
 
-    if (flush != Z_FINISH && err == Z_BUF_ERROR
-	    && z->stream.avail_out > 0) {
-	z->flags |= ZSTREAM_FLAG_IN_STREAM;
+    if (flush != Z_FINISH && err == Z_BUF_ERROR && z->stream.avail_out > 0) {
+        z->flags |= ZSTREAM_FLAG_IN_STREAM;
+
+        /* Z_BUF_ERROR with avail_out > 0 is a valid state - don't raise error */
+        err = Z_OK;
     }
 
     zstream_reset_input(z);