xcb_conn: Add a check for NULL to silence a UBSan runtime error

xcb_conn.c:314:60: runtime error: applying zero offset to null pointer

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/xcb_conn.c b/src/xcb_conn.c
index 3084c18..8f91f43 100644
--- a/src/xcb_conn.c
+++ b/src/xcb_conn.c
@@ -310,9 +310,11 @@ static int write_vec(xcb_connection_t *c, struct iovec **vector, int *count)
         int cur = (*vector)->iov_len;
         if(cur > n)
             cur = n;
-        (*vector)->iov_len -= cur;
-        (*vector)->iov_base = (char *) (*vector)->iov_base + cur;
-        n -= cur;
+        if(cur) {
+            (*vector)->iov_len -= cur;
+            (*vector)->iov_base = (char *) (*vector)->iov_base + cur;
+            n -= cur;
+        }
         if((*vector)->iov_len)
             break;
     }