bug: Assign values once in bug_get_file_line()

Set bug_get_file_line()'s output parameter values directly rather than
first nullifying them and then conditionally setting new values.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210318143311.839894-4-ascull@google.com