perf probe: Fix NULL pointer dereference in convert_variable_location()

If we just check whether the variable can be converted, 'tvar' should be
a null pointer. However, the null pointer check is missing in the
'Constant value' execution path.

The following cases can trigger this problem:

	$ cat test.c
	#include <stdio.h>

	void main(void)
	{
	        int a;
	        const int b = 1;

	        asm volatile("mov %1, %0" : "=r"(a): "i"(b));
	        printf("a: %d\n", a);
	}

	$ gcc test.c -o test -O -g
	$ sudo ./perf probe -x ./test -L "main"
	<main@/home/lhf/test.c:0>
	      0  void main(void)
	         {
	      2          int a;
	                 const int b = 1;

	                 asm volatile("mov %1, %0" : "=r"(a): "i"(b));
	      6          printf("a: %d\n", a);
	         }

	$ sudo ./perf probe -x ./test -V "main:6"
	Segmentation fault

The check on 'tvar' is added. If 'tavr' is a null pointer, we return 0
to indicate that the variable can be converted. Now, we can successfully
show the variables that can be accessed.

	$ sudo ./perf probe -x ./test -V "main:6"
	Available variables at main:6
	        @<main+13>
	                char*   __fmt
	                int     a
	                int     b

However, the variable 'b' cannot be tracked.

	$ sudo ./perf probe -x ./test -D "main:6 b"
	Failed to find the location of the 'b' variable at this address.
	 Perhaps it has been optimized out.
	 Use -V with the --range option to show 'b' location range.
	  Error: Failed to add events.

This is because __die_find_variable_cb() did not successfully match
variable 'b', which has the DW_AT_const_value attribute instead of
DW_AT_location. We added support for DW_AT_const_value in
__die_find_variable_cb(). With this modification, we can successfully
track the variable 'b'.

	$ sudo ./perf probe -x ./test -D "main:6 b"
	p:probe_test/main_L6 /home/lhf/test:0x1156 b=\1:s32

Fixes: 66f69b21

 ("perf probe: Support DW_AT_const_value constant value")
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Frank Ch. Eigler <fche@redhat.com>
Cc: Jianlin Lv <jianlin.lv@arm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Zhang Jinhao <zhangjinhao2@huawei.com>
http://lore.kernel.org/lkml/20210601092750.169601-1-lihuafei1@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>