contrib: fix var unbound issue on 'mt' with older bash

contrib/mt

@@ -76,7 +76,8 @@ if [ $# -gt 0 ]; then
       if [ -e $i ]; then
          fname=${i##*/}
          # build list of unique file names
-         if [[ ! " ${testfnames[*]} " =~ " $fname " ]]; then
+         if [ ${#testfnames[@]} -eq 0 ]\
+            || [[ ! " ${testfnames[*]} " =~ " $fname " ]]; then
             testfnames+=($fname)
          fi
       fi