notcurses_init: properly check flags for unknowns

src/lib/notcurses.c

@@ -735,7 +735,7 @@ notcurses* notcurses_init(const notcurses_options* opts, FILE* outfp){
     fprintf(stderr, "Provided an illegal negative margin, refusing to start\n");
     return NULL;
   }
-  if(opts->flags > NCOPTION_NO_ALTERNATE_SCREEN){
+  if(opts->flags > (NCOPTION_NO_ALTERNATE_SCREEN << 1u)){
     fprintf(stderr, "Provided an illegal Notcurses option, refusing to start\n");
     return NULL;
   }

src/poc/blitters.c

@@ -16,6 +16,9 @@ int main(int argc, char** argv){
     .flags = NCOPTION_INHIBIT_SETLOCALE | NCOPTION_NO_ALTERNATE_SCREEN,
   };
   struct notcurses* nc = notcurses_init(&nopts, NULL);
+  if(nc == NULL){
+    return EXIT_FAILURE;
+  }
   struct ncplane* std = notcurses_stdplane(nc);
   // FIXME not all work yet; this subset does
   const int blitters[] = {