strbuf: replace strbuf_grow() in strbuf_attach() with BUG() check

strbuf_attach() documents that alloc must be larger than len, as the
buffer must have room for the NUL terminator. Replace the strbuf_grow(sb, 0)
call, which was silently reallocating when alloc <= len, with an explicit
BUG() to enforce this contract and write the NUL terminator directly.

Signed-off-by: Vaidas Pilkauskas <vaidas.pilkauskas@shopify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: vaidas-shopify

strbuf.c

@@ -95,11 +95,12 @@ char *strbuf_detach(struct strbuf *sb, size_t *sz)
 
 void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
 {
+	if (alloc <= len)
+		BUG("alloc must be larger than len");
 	strbuf_release(sb);
 	sb->buf   = buf;
 	sb->len   = len;
 	sb->alloc = alloc;
-	strbuf_grow(sb, 0);
 	sb->buf[sb->len] = '\0';
 }