lexer: Fix implementation of lex_force_string_or_id().
[pspp] / tests / language / control /
drwxr-xr-x   ..
-rw-r--r-- 2463 do-if.at
-rw-r--r-- 3535 do-repeat.at
-rw-r--r-- 3905 loop.at
-rw-r--r-- 1257 temporary.at