Browsing Brazilian Portuguese translation

Some of the back ends can't deal with non-positive line numbers.
Besides, it's silly.  GCC however will generate a line number of
zero when it is pre-processing builtins for assembler-with-cpp files:

# 0 "<built-in>"

We do not want to barf on this, especially since such files are used
in the GCC and GDB testsuites.  So we check for negative line numbers
rather than non-positive line numbers.