Fix Galois field operations and add reference counting for table initialization

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.

Author: fferrandis

src/builtin/rs_vand/liberasurecode_rs_vand.c

@@ -370,7 +370,7 @@ void region_multiply(char *from_buf, char *to_buf, int mult, int xor, int blocks
 
     if (trailing_bytes == 1) {
       i = blocksize - 1;
-      to_buf[i] = to_buf[i] ^ (char)rs_galois_mult(from_buf[i], mult);
+      to_buf[i] = to_buf[i] ^ (char)rs_galois_mult((unsigned char)from_buf[i], mult);
     }
   } else {
     for (i = 0; i < adj_blocksize; i++) {
@@ -379,7 +379,7 @@ void region_multiply(char *from_buf, char *to_buf, int mult, int xor, int blocks
 
     if (trailing_bytes == 1) {
       i = blocksize - 1;
-      to_buf[i] = (char)rs_galois_mult(from_buf[i], mult);
+      to_buf[i] = (char)rs_galois_mult((unsigned char)from_buf[i], mult);
     }
   }
 }

src/builtin/rs_vand/rs_galois.c

@@ -44,31 +44,45 @@
 int *log_table = NULL;
 int *ilog_table = NULL;
 int *ilog_table_begin = NULL;
+static int init_counter = 0;
 
-void rs_galois_init_tables()
-{
-  log_table = (int*)malloc(sizeof(int)*FIELD_SIZE);
-  ilog_table_begin = (int*)malloc(sizeof(int)*FIELD_SIZE*3);
+void rs_galois_init_tables() {
+  if (init_counter++ > 0) {
+    /* already initialized */
+    return;
+  }
+  log_table = (int *)malloc(sizeof(int) * FIELD_SIZE);
+  ilog_table_begin = (int *)malloc(sizeof(int) * FIELD_SIZE * 3);
   int i = 0;
   int x = 1;
 
   for (i = 0; i < GROUP_SIZE; i++) {
     log_table[x] = i;
     ilog_table_begin[i] = x;
     ilog_table_begin[i + GROUP_SIZE] = x;
-    ilog_table_begin[i + (GROUP_SIZE*2)] = x;
+    ilog_table_begin[i + (GROUP_SIZE * 2)] = x;
     x = x << 1;
     if (x & FIELD_SIZE) {
-      x ^= PRIM_POLY; 
+      x ^= PRIM_POLY;
     }
   }
   ilog_table = &ilog_table_begin[GROUP_SIZE];
 }
 
-void rs_galois_deinit_tables()
-{
-  free(log_table);
-  free(ilog_table_begin);
+void rs_galois_deinit_tables() {
+  init_counter--;
+  if (init_counter < 0) {
+    /* deinit when not initialized?? */
+    init_counter = 0;
+  } else if (init_counter > 0) {
+    /* still at least one desc using it */
+    return;
+  } else {
+    free(log_table);
+    log_table = NULL;
+    free(ilog_table_begin);
+    ilog_table_begin = NULL;
+  }
 }
 
 int rs_galois_mult(int x, int y)