udev-rules: fix matching of token types that support alternative patterns

udev-rules: fix matching of token types that support alternative patterns

For those token types that support matching of alternative patterns,
their token values are interpreted as nulstr, so make sure the parser
does the right thing and makes these token values terminated by two
subsequent NULs so they could be safely interpreted as nulstr.

Before this fix, the following rules would result to "echo foo" invocation:
  ENV{foo}=", RUN"
  ENV{foo}=="bar", RUN+="echo foo"
because the value of `ENV{foo}` is treated as nulstr, and it used to match
against alternative patterns, in this case `bar`, `, RUN`, and `="echo foo`.

Fixes: 25de7aa7b90c ("udev: modernize udev-rules.c")