Fix Galois field operations and add reference counting for table init…

[fferrandis]

…ialization

This commit addresses two important improvements to the RS Vandermonde implementation:

1. Type safety fix in region_multiply():

   • Cast from_buf[i] to unsigned char before passing to rs_galois_mult()
   • Ensures correct behavior when processing signed char values in Galois field arithmetic operations
   • Applied to both XOR and non-XOR code paths

2. Reference counting for Galois table initialization:

   • Add init_counter to track multiple initialization/deinitialization calls
   • Prevent double initialization of log_table and ilog_table
   • Implement proper reference counting in rs_galois_deinit_tables() to only free tables when all references are released
   • Set pointers to NULL after freeing to prevent use-after-free issues

These changes improve robustness and prevent potential crashes from improper table lifecycle management.

[fferrandis]

/approve