Optimize wmem_strbuf_grow.

As suggested by Anders, in the case of repeated calls to wmem_strbuf_append_c
(and other functions which append very little data) the growth check was a
substantial portion of the over-all running time. By short-circuiting the check
in the case where growth isn't needed (as opposed to letting it fall-through
naturally) we shave ~25% off the cost of such repeated calls in my benchmarks.

The function (wmem_strbuf_grow) is inline, so the compiler should be able to
optimize each caller individually for the short-circuit.

Change-Id: I76419020f4d8fa675906eb77798969b6c61c7732
Reviewed-on: https://code.wireshark.org/review/1467
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/wmem/wmem_strbuf.c

@@ -106,6 +106,14 @@ wmem_strbuf_grow(wmem_strbuf_t *strbuf, const gsize to_add)
 {
     gsize  new_alloc_len, new_len;
 
+    /* short-circuit for efficiency if we have room already; greatly speeds up
+     * repeated calls to wmem_strbuf_append_c and others which grow a little bit
+     * at a time.
+     */
+    if (WMEM_STRBUF_ROOM(strbuf) >= to_add) {
+        return;
+    }
+
     new_alloc_len = strbuf->alloc_len;
     new_len = strbuf->len + to_add;