Resolve some TODO comments

- `out_PushSection` should not set `currentSection` to NULL because
  PUSHS, PUSHC, and PUSHO consistently keep the current section,
  charmap, and options, even though the stack has been pushed.

- `Callback__FILE__` does not need to assert that `fileName` is not
  empty because `__FILE__`'s value is quoted, and can safely be empty.

- `YY_FATAL_ERROR` and `YYLMAX` are not needed since the lexer is
  not generated with flex.

src/asm/section.c

@@ -902,7 +902,6 @@ void out_PushSection(void)
 	sect->offset = curOffset;
 	sect->next = sectionStack;
 	sectionStack = sect;
-	/* TODO: maybe set current section to NULL? */
 }
 
 void out_PopSection(void)