Allow wmem_strbuf_append_len() to append strings beginning with NUL byte

wmem_strbuf_append_len is for appending strings with embedded nulls,
so it shouldn't return early if the first byte of the string passed in
is null. The check for the string itself being null and the append_len
being zero is sufficient.

epan/wmem/wmem_strbuf.c

@@ -152,7 +152,7 @@ void
 wmem_strbuf_append_len(wmem_strbuf_t *strbuf, const gchar *str, gsize append_len)
 {
 
-    if (!append_len || !str || str[0] == '\0') {
+    if (!append_len || !str) {
         return;
     }