[ng] check: fix incompatibility with Solaris nawk

[ng] check: fix incompatibility with Solaris nawk

On Solaris 10, the parallel testsuite harness could fail with errors
like this when /usr/bin/nawk was the selected awk program.

  nawk: next is illegal inside a function at source line 1 in \
  function input_error; context is:
      function input_error(file) { ... close_current(); next; >>> } <<<

* lib/am/parallel-tests.am (am__create_global_log): In the awk script
defined by this function, avoid use of the 'next' directive inside a
function.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>