tests/tools_test: Use readlink() properly

Turns out that `readlink()` does not NULL-terminate the string and since
it resides on the stack we may end up with some junk:

  `/proc/self/exe point to /opt/igt/libexec/igt-gpu-tools/tools_test7<CD>\t<8A>^?`

That in turn confuses `dirname()` and we end up doing `chdir("/")`,
which explain the sporadic failures of this test where it was not able
to locate the tools.

Let's zero out the variable first and allow `readlink()` to write at
most `sizeof()-1` bytes to it, so it is always properly terminated.

Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Issue: #12


Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

tests/tools_test.c

@@ -77,7 +77,8 @@ static bool chdir_to_tools_dir(void)
 	igt_info("Failed to cd to %s\n", TOOLS);
 
 	/* Try TOOLS and install dir relative to test binary */
-	if (readlink("/proc/self/exe", path, sizeof(path)) > 0) {
+	memset(path, 0, sizeof(path)); /* readlink() does not append NUL */
+	if (readlink("/proc/self/exe", path, sizeof(path)-1) > 0) {
 		igt_info("/proc/self/exe point to %s, going to dirname()\n", path);
 		chdir(dirname(path));
 	}