Use delete instead of xfree for varobj_item

Use delete instead of xfree for varobj_item

In commit 2f408ec (Use ui_file_as_string throughout more), we start to
new varobj_item,

> -  vitem = XNEW (struct varobj_item);
> +  vitem = new varobj_item ();

but we still use xfree.  This causes some ASAN errors,

-var-update container^M
=================================================================^M
^[[1m^[[31m==20660==ERROR: AddressSanitizer: alloc-dealloc-mismatch (operator new vs free) on 0x602000090c10^M
^[[1m^[[0m    #0 0x2baa77d03631 in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x54631)^M
    #1 0x80e0c8 in xfree(void*) /home/yao/SourceCode/gnu/gdb/git/gdb/common/common-utils.c:100^M
    #2 0xc13670 in varobj_clear_saved_item /home/yao/SourceCode/gnu/gdb/git/gdb/varobj.c:727^M
    #3 0xc13957 in update_dynamic_varobj_children /home/yao/SourceCode/gnu/gdb/git/gdb/varobj.c:752^M
    #4 0xc1841c in varobj_update(varobj**, int) /home/yao/SourceCode/gnu/gdb/git/gdb/varobj.c:1699^M
    #5 0x5a2bf7 in varobj_update_one /home/yao/SourceCode/gnu/gdb/git/gdb/mi/mi-cmd-var.c:712^M
    #6 0x5a2a41 in mi_cmd_var_update(char*, char**, int) /home/yao/SourceCode/gnu/gdb/git/gdb/mi/mi-cmd-var.c:695^
........
^M
^[[1m^[[32m0x602000090c10 is located 0 bytes inside of 16-byte region [0x602000090c10,0x602000090c20)^M
^[[1m^[[0m^[[1m^[[35mallocated by thread T0 here:^[[1m^[[0m^M
    #0 0x2baa77d0415f in operator new(unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x5515f)^M
    #1 0x63613e in py_varobj_iter_next /home/yao/SourceCode/gnu/gdb/git/gdb/python/py-varobj.c:112^M
    #2 0xc13b89 in update_dynamic_varobj_children /home/yao/SourceCode/gnu/gdb/git/gdb/varobj.c:776^M
    #3 0xc1841c in varobj_update(varobj**, int) /home/yao/SourceCode/gnu/gdb/git/gdb/varobj.c:1699^M
    #4 0x5a2bf7 in varobj_update_one /home/yao/SourceCode/gnu/gdb/git/gdb/mi/mi-cmd-var.c:712^M
    #5 0x5a2a41 in mi_cmd_var_update(char*, char**, int) /home/yao/SourceCode/gnu/gdb/git/gdb/mi/mi-cmd-var.c:695^M

gdb:

2017-02-23  Yao Qi  <yao.qi@linaro.org>

	* varobj.c (varobj_clear_saved_item): Use delete instead of
	xfree.
	(update_dynamic_varobj_children): Likewise.