Clear variable "coredump_var_addr" before using it on gdb.base/coredump-filter.exp

Clear variable "coredump_var_addr" before using it on gdb.base/coredump-filter.exp

This commit is a continuation of the fix committed on:

  commit 8cd8f2f8ac49276437b7da37f275706ea1c1c925
  Author: Sergio Durigan Junior <sergiodj@redhat.com>
  Date:   Mon Apr 13 02:40:08 2015 -0400

      Rename variable "addr" to "coredump_var_addr" in gdb.base/coredump-filter.exp

Pedro pointed out that this fix was not complete, because the
testsuite could be run several times in a row (for example), which
means that it is not enough to just make the variable name unique: it
also needs to be cleared out if it is global.

This commit does that.  It is actually just a commit made to make
things totally correct; this specific test does not fail if you run it
several times in a row.

gdb/testsuite/ChangeLog:
2015-04-26  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/coredump-filter.exp: Clear variable "coredump_var_addr"
	before using it.