"false" could not be converted to a pointer type. Changed it to nullptr.
Also changed a few other "return 0" to "return nullptr".

Change-Id: Idd4573d4627bb89f1b79e3b21de5af5bd6063916
Reviewed-on: https://swiftshader-review.googlesource.com/7357Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>