Fix cbuffer initialization

If no data was appended, cbuf_get() would return a pointer to
uninitialized memory. Now it returns a pointer to a string of zero
length.

src/util/cbuffer.c

@@ -36,6 +36,7 @@ extern struct cbuffer* cbuf_create(size_t capacity)
 	buf->size = 0;
 	buf->flags = 0;
 	buf->buf = hub_malloc(capacity + 1);
+	buf->buf[0] = '\0';
 	return buf;
 }