lexer: Fix use-after-free error in lex_source_get_lookahead().

lexer: Fix use-after-free error in lex_source_get_lookahead().

This code used local variable 'out' as if its value stayed the same from
one iteration of the loop to the next, but in fact its scope meant that
it became indeterminate on each new iteration.  This commit fixes the
problem by moving its declaration to an outer scope.

Thanks to John Darrington for reporting the problem.