pipe-loader: check if winsys.name is non-null prior to strcmp

pipe-loader: check if winsys.name is non-null prior to strcmp

In theory this wouldn't be an issue, as we'll find the correct name and
break out of the loop before we hit the sentinel.

Let's fix this and avoid issues in the future.

Spotted by Coverity (CID 1339869, 1339870, 1339871)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>