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.