Clean up ast_check_digits

Clean up ast_check_digits

The code was originally copied from the is_int() function in the AEL
code. wdoekes pointed out that the function should take a const char*
and that their was an unneeded variable. This is now fixed.
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Merged revisions 341529 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 341530 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@341533 65c4cc65-6c06-0410-ace0-fbb531ad65f3