checkAPI: error out on empty filelist

This will help detect some bogus checkapi runs in the future.

Change-Id: I43345e3a0a5471e9655f34e36ccf1562d6526758
Reviewed-on: https://code.wireshark.org/review/23946
Reviewed-by: Michael Mann <mmann78@netscape.net>

tools/checkAPIs.pl

@@ -2126,6 +2126,8 @@ if ("$filenamelist" ne "") {
         close(FC);
 }
 
+die "no files to process" unless (scalar @filelist);
+
 # Read through the files; do various checks
 while ($_ = pop @filelist)
 {