[rtext] Don't return default font if LoadFontEx fails

[Peter0x44]

It is currently impossible to check a font loaded successfully with IsFontReady because LoadFontEx will always return a valid font.

DrawTextEx has this check:
if (font.texture.id == 0) font = GetFontDefault(); // Security check in case of not valid font

So anyone relying on the default font as a fallback for fonts failing to load should still be covered.

[CrackedPixel]

@Peter0x44 do you think something like this should be applied to loading shaders as well?

[raysan5]

@Peter0x44 @CrackedPixel raylib has some fallbacks for convenience, change this behaviour could imply reviewing several assets loading functions... It's not a one-function trivial change.

[Peter0x44]

@raysan5 I think it is.
Both DrawTextEx and MeasureTextEx have checks for passing invalid fonts to them, and just use the default font instead.
Are there any others I need to be aware of?

[Peter0x44]

About the shaders, I didn't think about reviewing them, but it can be done at a later time.
i believe this change is self-contained and should be okay.

[raysan5]

Just for completion, here it is a list with the result of loading several types of assets (from files) and failing doing it:

• Image: Returns invalid image, image.data = NULL
• Texture2D: Returns invalid texture, texture.id = 0
• Font: Fallback to GetFontDefault() --> Return invalid font -> Does it CRASH anything?
• Mesh: Returns invalid vertex data, mesh.vertex = NULL, mesh.vaoId = 0
• Shader: Fallback to default shader RLGL.State.defaultShaderId
• Material: Fallback to LoadMaterialDefault()
• Wave: Returns invalid wave, wave.data = NULL
• Sound: Returns invalid sound, sounds.stream.buffer = NULL
• Music: Returns invalid music, music.stream.buffer = NULL

@Peter0x44 Please, could you review if failing on font loading, with no fallback, any example crashes?