fix two my_setenv/my_clearenv bugs

RT #117121. This commit fixes two bugs. First, some old glibc's can crash
if unsetenv() is called with a null environ.

Secondly, the code in Perl_my_clearenv(), when freeing env vars with a
name longer than 80 chars, reallocs a tmp buffer, but didn't
update the buf lencausing this to fail under valgrind:

    $ENV{"a"x 90}=1; local %ENV;
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