Fix crash vulnerabilities in path animation and layer parent handling

Two stability issues have been addressed:

1. PathData specialization: Added missing empty-frames guard in the
   Property<PathData>::value() method at lottiemodel.h. This prevents
   a null pointer dereference when animation keyframes are discarded
   during parsing, leaving the frames array empty.

2. Layer parent recursion: Added depth limiting to Layer::matrix() at
   lottieitem.cpp to prevent stack overflow from cyclic parent layer
   references. The method now uses an internal overload with a depth
   counter that returns an identity matrix when exceeding 64 levels.

Files changed:
  - src/lottie/lottiemodel.h: Added empty check before accessing frames
  - src/lottie/lottieitem.h: Declared overloaded matrix() with depth param
  - src/lottie/lottieitem.cpp: Implemented depth-limited matrix traversal