strbuf: pass correct alloc to strbuf_attach() in strbuf_reencode()

reencode_string_len() allocates len+1 bytes (including the NUL) and
returns the string length in len. strbuf_reencode() was calling
strbuf_attach(sb, out, len, len), so alloc was one byte too small.

strbuf_attach() then calls strbuf_grow(sb, 0). With alloc < len+1,
ALLOC_GROW always reallocates, so we reallocated immediately after
attach even when the strbuf was not extended further. Pass len+1 as
the alloc argument so the existing buffer is reused and the
reallocation is avoided.

Signed-off-by: Vaidas Pilkauskas <vaidas.pilkauskas@shopify.com>

Author: vaidas-shopify

strbuf.c

@@ -168,7 +168,7 @@ int strbuf_reencode(struct strbuf *sb, const char *from, const char *to)
 	if (!out)
 		return -1;
 
-	strbuf_attach(sb, out, len, len);
+	strbuf_attach(sb, out, len, len + 1);
 	return 0;
 }