Allocations.to_hash should only contain classes

Sometimes, for whatever reason, this output may contain instances of
objects. For example, in a test script I ended up with an Enumerable
instance in the output (which then triggers an error as Enumerable
doesn't respond to #hash). I have no idea how on earth an Enumerable can
1) get initialized since it's a module 2) ends up in the output.

ext/liballocations/liballocations.c

@@ -56,7 +56,11 @@ void freeobj_callback(VALUE tracepoint, void* data) {
  * Copies every value in an st_table to a given Ruby Hash.
  */
 static int each_count(st_data_t key, st_data_t value, st_data_t hash_ptr) {
-    rb_hash_aset((VALUE) hash_ptr, (VALUE) key, INT2NUM(value));
+    VALUE vkey = (VALUE) key;
+
+    if ( rb_obj_is_kind_of(key, rb_cClass) == Qtrue ) {
+        rb_hash_aset((VALUE) hash_ptr, vkey, INT2NUM(value));
+    }
     return ST_CONTINUE;
 }