strbuf: make sure buffer is zero-terminated

strbuf_init does not zero-terminate the initial buffer when hint is
non-zero. Fix this so we can rely on the string to be zero-terminated
even if we haven't filled it with anything yet.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -30,8 +30,10 @@ void strbuf_init(struct strbuf *sb, size_t hint)
 {
 	sb->alloc = sb->len = 0;
 	sb->buf = strbuf_slopbuf;
-	if (hint)
+	if (hint) {
 		strbuf_grow(sb, hint);
+		sb->buf[0] = '\0';
+	}
 }
 
 void strbuf_release(struct strbuf *sb)