This commit fixes a regression introduced by the new $_isvoid convenience

This commit fixes a regression introduced by the new $_isvoid convenience
function.  It adds a check for $_isvoid during the test of "show convenience"
output.

gdb/testsuite/
2013-09-17  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/defaults.exp (<show_conv_list>): Add check for $_isvoid
	convenience function.