Make Texture.add() firstFrame check more explicit (Fix issue #4088)

[Cirras]

Fixes issue #4088 where a SpriteSheet created from a trimmed texture atlas frame returns the second frame when the first frame is requested.

This PR

• Fixes a bug

Describe the changes below:
-Made simple change to Texture.add() that will prevent the second frame from being selected as the firstFrame in the event that __BASE texture does not exist.

[photonstorm]

This is now fixed (albeit indirectly) because the sprite sheet parser now automatically creates a __BASE entry, where it didn't before. However, I like this checking approach better anyway, so am going to merge it anyway.