lexer: Fix negative 'n0' and 'n1' in lex_source_contains_macro_call().
[pspp] / src /
drwxr-xr-x   ..
-rw-r--r-- 1675 automake.mk
drwxr-xr-x - data
drwxr-xr-x - language
drwxr-xr-x - libpspp
drwxr-xr-x - math
drwxr-xr-x - output
drwxr-xr-x - ui