Correctly handle reading locations from serialized diagnostics

When reading a serialized diagnostic location with no file ID, we were
failing to increment the cursor past the rest of the location. This
would lead to the flags and category always appearing blank in such
diagnostics.

This changes the function to unconditionally increment the cursor and
updates the test to check for the correct output instead of testing
that we were doing this wrong. I've also updated the error check to
check for the correct number of fields.