efm_perl: fix warning about uninitialized $opt_I

$ perl syntax_checkers/efm_perl.pl -c -w /dev/null
Use of uninitialized value $opt_I in split at syntax_checkers/efm_perl.pl line 94.
efm_perl: add support for checking - (stdin)

In simple case it just miss warnings:
  $ echo '$a=5;' | perl syntax_checkers/efm_perl.pl -c -w -
  (nothing, while it should be this:)
  W:-:1:Name "main::a" used only once: possible typo
In complex cases with some other errors it show crazy warnings
in crazy line numbers.
Use case when efm_perl run on STDIN can be found in issue#261.
Merge pull request #428 from powerman/efm_perl

Efm perl: few fixed