glGetFragDataLocation() should only query fragment output varyings.
Also fix checking for too many varyings.

Change-Id: I0981fb69a28762b6d41a22e0544d63c76510000c
Reviewed-on: https://swiftshader-review.googlesource.com/16209Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>