Fix coredump-filter.exp by correctly unsetting array

Fix coredump-filter.exp by correctly unsetting array

In my last commit to make gdb.base/coredump-filter.exp be more robust
regarding using arrays in the global namespace, I cleared the
"coredump_var_addr" array like this:

  set coredump_var_addr ""
  # use coredump_var_addr as an array...

This causes DejaGNU to complain because the variable is first set as
non-array, and the used as an array.  The correct way to do this is to
unset the variable using:

  unset -nocomplain coredump_var_addr
  # use coredump_var_addr as an array...

The "-nocomplain" part is necessary because if the variable doesn't
exist "unset" will not error.

Tested on Fedora 20 x86_64.

gdb/testsuite/ChangeLog:
2015-05-08  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/coredump-filter.exp: Correctly unset
	"coredump_var_addr" array.