Fix error logging spew

Prior to this commit, the error log from the websesrver would be filled with lines like:

   [Thu May 08 04:05:55 2014] [error] [client 192.168.1.3] Use of uninitialized
       value $c in substitution (s///) at lib/tumble.pm line 166.

This commit fixes it so the substitution will not be attempted unless $c actually contains something.