strbuf.h: use BUG(...) not die("BUG: ...")

In 7141efab24 (strbuf: clarify assertion in strbuf_setlen(),
2011-04-27) this 'die("BUG: "' invocation was added with the rationale
that strbuf.c had existing users doing the same, but those users were
later changed to use BUG() in 033abf97fc (Replace all die("BUG: ...")
calls by BUG() ones, 2018-05-02). Let's do the same here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -160,7 +160,7 @@ void strbuf_grow(struct strbuf *sb, size_t amount);
 static inline void strbuf_setlen(struct strbuf *sb, size_t len)
 {
 	if (len > (sb->alloc ? sb->alloc - 1 : 0))
-		die("BUG: strbuf_setlen() beyond buffer");
+		BUG("strbuf_setlen() beyond buffer");
 	sb->len = len;
 	if (sb->buf != strbuf_slopbuf)
 		sb->buf[len] = '\0';