Use xfree rather than delete for varobj_iter

Use xfree rather than delete for varobj_iter

ASAN reports an error,

-var-create container @ c^M
=================================================================^M
^[[1m^[[31m==21639==ERROR: AddressSanitizer: alloc-dealloc-mismatch (malloc vs operator delete) on 0x6030000805c0^M
^[[1m^[[0m    #0 0x7f2449b01b2a in operator delete(void*) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x99b2a)^M
    #1 0xbb601d in update_dynamic_varobj_children ../../binutils-gdb/gdb/varobj.c:794^M
    #2 0xbb6556 in varobj_get_num_children(varobj*) ../../binutils-gdb/gdb/varobj.c:854^M
    #3 0x580cb4 in print_varobj ../../binutils-gdb/gdb/mi/mi-cmd-var.c:61^M
    #4 0x58138b in mi_cmd_var_create(char*, char**, int) ../../binutils-gdb/gdb/mi/mi-cmd-var.c:145^M
    #5 0x5967ce in mi_cmd_execute ../../binutils-gdb/gdb/mi/mi-main.c:2301^M
    #6 0x594b05 in captured_mi_execute_command ../../binutils-gdb/gdb/mi/mi-main.c:2001
....
^M
^[[1m^[[32m0x6030000805c0 is located 0 bytes inside of 32-byte region [0x6030000805c0,0x6030000805e0)^M
^[[1m^[[0m^[[1m^[[35mallocated by thread T0 here:^[[1m^[[0m^M
    #0 0x7f2449b00602 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x98602)^M
    #1 0x7d1596 in xmalloc ../../binutils-gdb/gdb/common/common-utils.c:43^M
    #2 0x604176 in py_varobj_iter_new ../../binutils-gdb/gdb/python/py-varobj.c:159^M
    #3 0x6042da in py_varobj_get_iterator(varobj*, _object*) ../../binutils-gdb/gdb/python/py-varobj.c:198^M
    #4 0xbb5806 in varobj_get_iterator ../../binutils-gdb/gdb/varobj.c:720^M
    #5 0xbb5b9b in update_dynamic_varobj_children ../../binutils-gdb/gdb/varobj.c:758^M

gdb:

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

	* varobj-iter.h (varobj_iter_delete): Call xfree instead of
	delete.