mruby-compiler: fix uninitialized memory in realloc_pool_str()

when converting a shared/static string (IREP_TT_SSTR) to heap-allocated
(IREP_TT_STR), copy the original content to the new buffer.

previously, the original content was lost when allocating new memory,
leaving the first bytes uninitialized. this caused find_pool_str() to
read uninitialized memory via memcmp() when searching for duplicate
strings.

reported by OSS-Fuzz.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/codegen.c

@@ -1242,8 +1242,11 @@ static void
 realloc_pool_str(codegen_scope *s, mrb_irep_pool *p, mrb_int len)
 {
   char *str;
+  mrb_int olen = p->tt >> 2;  /* original length */
   if ((p->tt & 3) == IREP_TT_SSTR) { /* Check if it's a shared/static string */
+    const char *old = p->u.str;
     str = (char*)mrbc_malloc(len+1); /* Allocate new memory if it was shared */
+    memcpy(str, old, olen);  /* Copy original content */
   }
   else { /* It's already a heap-allocated string */
     str = (char*)p->u.str;