fix leak in gdb_environ

fix leak in gdb_environ

valgrind reports a leak when assigning a gdb_environ to another gdb_environ.
The memory allocated for the target gdb_environ env variables is not released.
The gdb_environ selftest reproduces the leak (see below).
Fix the leak by clearing the target gdb_environ before std::move-ing the
members.

Tested natively and re-running all tests under valgrind.

==3261873== 4,842 bytes in 69 blocks are definitely lost in loss record 6,772 of 6,839
==3261873==    at 0x483979B: malloc (vg_replace_malloc.c:393)
==3261873==    by 0x25A454: xmalloc (alloc.c:57)
==3261873==    by 0x7D1E4E: xstrdup (xstrdup.c:34)
==3261873==    by 0x7E2A51: gdb_environ::from_host_environ() (environ.cc:56)
==3261873==    by 0x66F1C8: test_reinit_from_host_environ (environ-selftests.c:78)
==3261873==    by 0x66F1C8: selftests::gdb_environ_tests::run_tests() (environ-selftests.c:285)
==3261873==    by 0x7EFC43: operator() (std_function.h:622)
=

Approved-By: Simon Marchi <simon.marchi@efficios.com>