Fix/work around a bug in parse_expr().

There was an obscure bug in parse_expr(), used to evaluate 'if df',
where it could skip past the end of the line marker.

If this happened inside an expanded macro, then after that was the
next line...
In other cases it might be worse.

Now that this is fixed, another check for line end junk can be enabled.

assemble.c

@@ -932,7 +932,7 @@ static int assemble(
                                                    suppressed
                                                    until .ENDC */
                     }
-                    return 1;                   /* TODO: CHECK_EOL */
+                    return CHECK_EOL;
 
                 case P_IFF:
                     if (last_cond < 0) {