(partially) Fix Bug#55227 Fix compiler warnings in innodb with gcc 4.6

Fix compiler warning:
trx/trx0sys.c: In function 'trx_sys_create_doublewrite_buf':
trx/trx0sys.c:244:15: error: variable 'new_block' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/trx/trx0sys.c

@@ -241,7 +241,9 @@ trx_sys_create_doublewrite_buf(void)
 {
 	buf_block_t*	block;
 	buf_block_t*	block2;
+#ifdef UNIV_DEBUG
 	buf_block_t*	new_block;
+#endif /* UNIV_DEBUG */
 	byte*	doublewrite;
 	byte*	fseg_header;
 	ulint	page_no;
@@ -344,7 +346,10 @@ start_again:
 			the page position in the tablespace, then the page
 			has not been written to in doublewrite. */
 
-			new_block = buf_page_get(TRX_SYS_SPACE, 0, page_no,
+#ifdef UNIV_DEBUG
+			new_block =
+#endif /* UNIV_DEBUG */
+			buf_page_get(TRX_SYS_SPACE, 0, page_no,
 				     RW_X_LATCH, &mtr);
 			buf_block_dbg_add_level(new_block,
 						SYNC_NO_ORDER_CHECK);