Get rid of warnings/exceptions at cleanup

After the encoding index instance variable is removed when all
instance variables are removed in `obj_free`, then `rb_str_free`
causes uninitialized instance variable warning and nil-to-integer
conversion exception.  Both cases result in object allocation
during GC, and crashes.

encoding.c

@@ -779,8 +779,18 @@ enc_get_index_str(VALUE str)
     if (i == ENCODING_INLINE_MAX) {
 	VALUE iv;
 
+#if 0
 	iv = rb_ivar_get(str, rb_id_encoding());
 	i = NUM2INT(iv);
+#else
+        /*
+         * Tentatively, assume ASCII-8BIT, if encoding index instance
+         * variable is not found.  This can happen when freeing after
+         * all instance variables are removed in `obj_free`.
+         */
+        iv = rb_attr_get(str, rb_id_encoding());
+        i = NIL_P(iv) ? ENCINDEX_ASCII : NUM2INT(iv);
+#endif
     }
     return i;
 }

string.c

@@ -181,6 +181,7 @@ VALUE rb_cSymbol;
 
 #define STR_HEAP_PTR(str)  (RSTRING(str)->as.heap.ptr)
 #define STR_HEAP_SIZE(str) ((size_t)RSTRING(str)->as.heap.aux.capa + TERM_LEN(str))
+/* TODO: include the terminator size in capa. */
 
 #define STR_ENC_GET(str) get_encoding(str)
 

test/ruby/test_encoding.rb

@@ -76,6 +76,9 @@ class TestEncoding < Test::Unit::TestCase
       assert_equal("0", format % 0)
       assert_equal(e, format.dup.encoding)
       assert_equal(e, (format*1).encoding)
+
+      assert_equal(e, (("x"*30).force_encoding(e)*1).encoding)
+      GC.start
     end;
   end