fix failure to report write error (ENOSPC) on fmemopen streams

src/stdio/fmemopen.c

@@ -63,7 +63,12 @@ static size_t mwrite(FILE *f, const unsigned char *buf, size_t len)
 	}
 	if (c->mode == 'a') c->pos = c->len;
 	rem = c->size - c->pos;
-	if (len > rem) len = rem;
+	if (len > rem) {
+		errno = ENOSPC;
+		f->wpos = f->wbase = f->wend = 0;
+		f->flags |= F_ERR;
+		len = rem;
+	}
 	memcpy(c->buf+c->pos, buf, len);
 	c->pos += len;
 	if (c->pos > c->len) {