more fmemopen null termination fixes

null termination is only added when current size grows.
in update modes, null termination is not added if it does not fit
(i.e. it is not allowed to clobber data).

these rules make very little sense, but that's how it goes..

src/stdio/fmemopen.c

@@ -54,9 +54,10 @@ static size_t mwrite(FILE *f, const unsigned char *buf, size_t len)
 	if (len > rem) len = rem;
 	memcpy(c->buf+c->pos, buf, len);
 	c->pos += len;
-	if (c->pos >= c->len) {
+	if (c->pos > c->len) {
 		c->len = c->pos;
-		c->buf[c->len==c->size ? c->len-1 : c->len] = 0;
+		if (c->len < c->size) c->buf[c->len] = 0;
+		else if ((f->flags&F_NORD) && c->size) c->buf[c->size-1] = 0;
 	}
 	return len;
 }