rexec: Avoid invalid free in rexec failure path

rexec: Avoid invalid free in rexec failure path

Commit "rexec: free argv array on failure" used __do_free_string_list
as a destructor for argv, which is an array of pointers to a single
buffer and not an array of pointers to independent buffers, which leads
to an attempt to free invalid pointer whenever argv has more than one
element.

Structure argv as one memory block and use __do_free as the destructor.

Signed-off-by: Petr Malat <oss@malat.biz>