strbuf: use valid pointer in strbuf_remove()

The fourth argument of strbuf_splice() is passed to memcpy(3), which is
not supposed to handle NULL pointers.  Let's be extra careful and use a
valid empty string instead.  It even shortens the source code. :)

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -187,7 +187,7 @@ void strbuf_insert(struct strbuf *sb, size_t pos, const void *data, size_t len)
 
 void strbuf_remove(struct strbuf *sb, size_t pos, size_t len)
 {
-	strbuf_splice(sb, pos, len, NULL, 0);
+	strbuf_splice(sb, pos, len, "", 0);
 }
 
 void strbuf_add(struct strbuf *sb, const void *data, size_t len)