strbuf_setlen(): do not barf on setting length of an empty buffer to 0

strbuf_setlen() expect to be able to NUL terminate the buffer,
but a completely empty strbuf could have an empty buffer with 0
allocation; both the assert() and the assignment for NUL
termination would fail.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -65,14 +65,17 @@ static inline void strbuf_swap(struct strbuf *a, struct strbuf *b) {
 static inline size_t strbuf_avail(struct strbuf *sb) {
 	return sb->alloc ? sb->alloc - sb->len - 1 : 0;
 }
+
+extern void strbuf_grow(struct strbuf *, size_t);
+
 static inline void strbuf_setlen(struct strbuf *sb, size_t len) {
-	assert (len < sb->alloc);
+	if (!sb->alloc)
+		strbuf_grow(sb, 0);
+	assert(len < sb->alloc);
 	sb->len = len;
 	sb->buf[len] = '\0';
 }
 
-extern void strbuf_grow(struct strbuf *, size_t);
-
 /*----- content related -----*/
 extern void strbuf_rtrim(struct strbuf *);