Make VSB_bcat() take a ssize_t to be able to assert non-negative lengths.

Spotted by:	FlexeLint

include/vsb.h

@@ -61,7 +61,7 @@ struct vsb	*VSB_new(struct vsb *, char *, int, int);
 #define		 VSB_new_auto()				\
 	VSB_new(NULL, NULL, 0, VSB_AUTOEXTEND)
 void		 VSB_clear(struct vsb *);
-int		 VSB_bcat(struct vsb *, const void *, size_t);
+int		 VSB_bcat(struct vsb *, const void *, ssize_t);
 int		 VSB_cat(struct vsb *, const char *);
 int		 VSB_printf(struct vsb *, const char *, ...)
 	__v_printflike(2, 3);

lib/libvarnish/vsb.c

@@ -276,13 +276,16 @@ VSB_put_byte(struct vsb *s, int c)
  * Append a byte string to an vsb.
  */
 int
-VSB_bcat(struct vsb *s, const void *buf, size_t len)
+VSB_bcat(struct vsb *s, const void *buf, ssize_t len)
 {
 	assert_VSB_integrity(s);
 	assert_VSB_state(s, 0);
 
+	assert(len >= 0);
 	if (s->s_error != 0)
 		return (-1);
+	if (len == 0)
+		return (0);
 	_vsb_indent(s);
 	if (len > VSB_FREESPACE(s)) {
 		if (VSB_extend(s, len - VSB_FREESPACE(s)) < 0)