[rmodels] Address review feedback on glTF joint offset handling
Resolve the open review points raised on PR #5876 for the per-joint
extOffset precompute in LoadModelAnimationsGLTF.
* NULL check: validate the extOffset RL_MALLOC result before use, which
was previously missing.
* Fail-fast handling: detect the allocation failure at its source and
abort animation loading (log a warning, free resources, return NULL)
instead of propagating a NULL pointer into the per-frame loop.
* Brace formatting: expand the collapsed one-line joint-match check
(if (...) { isJoint = true; break; }) into raylib's standard Allman
brace style.
* Readability: break the deeply nested MatrixMultiply(MatrixMultiply(...))
into named nodeScale/nodeRotation/nodeTranslation/nodeTransform locals,
mirroring the existing S/R/T composition later in the function.
* Spacing/line breaks: add blank lines within the precompute block to
match the surrounding code style.
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