strbuf: clarify assertion in strbuf_setlen()

Commit a8f3e2219 introduced the strbuf_grow() call to strbuf_setlen() to
make ensure that there was at least one byte available to write the
mandatory trailing NUL, even for previously unallocated strbufs.

Then b315c5c0 added strbuf_slopbuf for the same reason, only globally for
all uses of strbufs.

Thus the strbuf_grow() call can be removed now.  This avoids readers of
strbuf.h from mistakenly thinking that strbuf_setlen() can be used to
extend a strbuf.

The following assert() needs to be changed to cope with the fact that
sb->alloc can now be zero, which is OK as long as len is also zero.  As
suggested by Junio, use the chance to convert it to a die() with a short
explanatory message.  The pattern of 'die("BUG: ...")' is already used in
strbuf.c.

This was the only assert() in strbuf.[ch], so assert.h doesn't have to be
included anymore either.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -3,8 +3,6 @@
 
 /* See Documentation/technical/api-strbuf.txt */
 
-#include <assert.h>
-
 extern char strbuf_slopbuf[];
 struct strbuf {
 	size_t alloc;
@@ -33,9 +31,8 @@ static inline size_t strbuf_avail(const struct strbuf *sb) {
 extern void strbuf_grow(struct strbuf *, size_t);
 
 static inline void strbuf_setlen(struct strbuf *sb, size_t len) {
-	if (!sb->alloc)
-		strbuf_grow(sb, 0);
-	assert(len < sb->alloc);
+	if (len > (sb->alloc ? sb->alloc - 1 : 0))
+		die("BUG: strbuf_setlen() beyond buffer");
 	sb->len = len;
 	sb->buf[len] = '\0';
 }