[rmodels] Ensure that GLTF buffers are deallocated before using them

[JeffM2501]

Check to see if a mesh buffer is allocated before reallocating it so we don't leak that buffer in cases where the gltf may have duplicate attributes at different resolutions.

Should fix issue #4996

[raysan5]

I'm afraid I'm not adding all those checks, two reasons:

1. Users are the owners of data and should be careful with it
2. If the pointers are freed but not zeroed, it will fail

[JeffM2501]

we will have to make a different check then. This is a legitimate memory leak.
The other option is to skip loading the attribute if one of the same type has already been loaded.
but if we do not do anything the library will leak memory.
The duplicate attributes are added by the model exporters, and are not controllable by the end user, so raylib must deal with this situation in a valid way.