Add slot size assertion to ar_force_convert_table

Co-authored-by: Luke Gruber <luke.gru@gmail.com>
Co-authored-by: Matt Valentine-House <matt@eightbitraptor.com>

Author: 3 people

hash.c

@@ -604,6 +604,7 @@ RHASH_AR_TABLE_SIZE_DEC(VALUE h)
 static inline void
 RHASH_AR_TABLE_CLEAR(VALUE h)
 {
+    RUBY_ASSERT(rb_gc_obj_slot_size(h) >= sizeof(struct RHash) + sizeof(ar_table));
     RBASIC(h)->flags &= ~RHASH_AR_TABLE_SIZE_MASK;
     RBASIC(h)->flags &= ~RHASH_AR_TABLE_BOUND_MASK;
 
@@ -719,6 +720,8 @@ ar_force_convert_table(VALUE hash, const char *file, int line)
         st_hash_t hashes[RHASH_AR_TABLE_MAX_SIZE];
         unsigned int bound, size;
 
+        RUBY_ASSERT(rb_gc_obj_slot_size(hash) >= sizeof(struct RHash) + sizeof(ar_table));
+
         // prepare hash values
         do {
             st_data_t keys[RHASH_AR_TABLE_MAX_SIZE];