parser: Pop namespace in repeatable nested

! In nested-name-specifier-repeatable, the previous active namespace
  (from another -repeatable or from -start) should be taken from the
  stack.

Signed-off-by: Max Reitz <xanclic@niwohlos.org>
specs: parserspec::ex5

+ This test involves namespaces.

Signed-off-by: Max Reitz <xanclic@niwohlos.org>