René Scharfe authored Apr 27, 2011 and Junio C Hamano committed Apr 27, 2011

Commit a8f3e221 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>