Update greedy.c

This code working absolutely fine.I Study this code then i found condition which is ignored .
If user gave you 0 or negative number its shows only one message then its compile only do Statement 
so i code two line at there i checked where given_amount is zero or less then zero then its gave you
Message "Number Should be greater then Zero EG:10\n:"
Merge pull request #1 from Milind300/master

Update greedy.c