java: don't assume jar is installed

Author: scop

completions/java

@@ -69,7 +69,7 @@ _java_classes()
                 COMPREPLY+=( $(unzip -lq "$i" "$cur*" 2>/dev/null | \
                     awk '$NF ~ /^-+$/ { flag=!flag; next };
                          flag && $NF ~ /^[^$]*\.class/ { print $NF }') )
-            else
+            elif type jar &>/dev/null; then
                 COMPREPLY+=( $(jar tf "$i" "$cur" | \
                     command grep '^[^$]*\.class$') )
             fi