Fix bug when using gzflush() with a very small buffer.

gzlib.c

@@ -308,8 +308,8 @@ int ZEXPORT gzbuffer(gzFile file, unsigned size) {
     /* check and set requested size */
     if ((size << 1) < size)
         return -1;              /* need to be able to double it */
-    if (size < 2)
-        size = 2;               /* need two bytes to check magic header */
+    if (size < 8)
+        size = 8;               /* needed to behave well with flushing */
     state->want = size;
     return 0;
 }