dx_getline: Fix EOF handling in standard C fallback.

dx_getline: Fix EOF handling in standard C fallback.

The return value of the function at end of input was not tested so of
course it does not work: in the standard C fallback, a mistake was
introduced during the import and it returns DX_GETLINE_ERROR instead
of the expected DX_GETLINE_EOF.

Easy enough to fix and update the test program to check this.