Treat unexpected .IF argument as true, not false. This appears

to be what the reference assembler does, at least it is necessary
for certain RSTS source files to assemble without error.

assemble.c

@@ -834,7 +834,7 @@ do_mcalled_macro:
                                 report(stack->top, "Bad .IF expression\n");
                                 list_value(stack->top, 0);
                                 free_tree(tvalue);
-                                ok = FALSE;     /* Pick something. */
+                                ok = TRUE;     /* Pick something. */
                             } else {
                                 unsigned        word;