[mucurses] Fix GCC 6 nonnull-compare errors

Remove null checks for arguments declared as nonnull.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/mucurses/windows.c

  * @ret rc	return status code
  */
 int delwin ( WINDOW *win ) {
-	if ( win == NULL )
-		return ERR;
-
 	/* I think we should blank the region covered by the window -
 	   ncurses doesn't do this, but they have a buffer, so they
 	   may just be deleting from an offscreen context whereas we
 WINDOW *derwin ( WINDOW *parent, int nlines, int ncols,
 	     		  	 int begin_y, int begin_x ) {
 	WINDOW *child;
-	if ( parent == NULL )
-		return NULL;
 	if ( ( child = malloc( sizeof( WINDOW ) ) ) == NULL )
 		return NULL;
 	if ( ( (unsigned)ncols > parent->width ) || 
  */
 WINDOW *dupwin ( WINDOW *orig ) {
 	WINDOW *copy;
-	if ( orig == NULL )
-		return NULL;
 	if ( ( copy = malloc( sizeof( WINDOW ) ) ) == NULL )
 		return NULL;
 	copy->scr = orig->scr;
  * @ret rc	return status code
  */
 int mvwin ( WINDOW *win, int y, int x ) {
-	if ( win == NULL )
-		return ERR;
 	if ( ( ( (unsigned)y + win->height ) > LINES ) ||
 	     ( ( (unsigned)x + win->width ) > COLS ) )
 		return ERR;
 WINDOW *subwin ( WINDOW *parent, int nlines, int ncols,
 			         int begin_y, int begin_x ) {
 	WINDOW *child;
-	if ( parent == NULL )
-		return NULL;
 	if ( ( child = malloc( sizeof( WINDOW ) ) ) == NULL )
 		return NULL;
 	child = newwin( nlines, ncols, begin_y, begin_x );