Tidy up variable declarations

In general, we should be declaring variables "local" when they apply
only below a certain function.  Make this happen.

Signed-off-by: John Keeping <john@keeping.me.uk>
Remove a superfluous assignment

current_insn is reset in the parsing loop after finalize_commit has been
called.  We don't need to reset it after processing a "base ..." line as
well.

Signed-off-by: John Keeping <john@keeping.me.uk>