Update RNNSplashScreen.m (#4230)

Fixed app opening on iphoneX family(X,XS,XSMAX,XR)
When you don't have a launch image and you have a Default-812h.png file in your xcassets
(you would get a black screen)

lib/ios/RNNSplashScreen.m

 			imageName = [imageName stringByAppendingString:@"-667h"];
 		else if (screenHeight == 736)
 			imageName = [imageName stringByAppendingString:@"-736h"];
+		else if (screenHeight == 812)
+			imageName = [imageName stringByAppendingString:@"-812h"];
 		
 		//xcassets LaunchImage files
 		UIImage *image = [UIImage imageNamed:imageName];