tools/test-runner: Fix errors reported by scan-build

tools/test-runner: Fix errors reported by scan-build

This patch fixes the errors reported by the scan-build.

tools/test-runner.c:315:2: warning: Null pointer passed to 1st parameter
expecting 'nonnull' [core.NonNullParamChecker]
        execve(argv[0], argv, qemu_envp);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

tools/test-runner.c:554:6: warning: Null pointer passed to 1st parameter
expecting 'nonnull' [core.NonNullParamChecker]
        if (chdir(home + 5) < 0) {
            ^~~~~~~~~~~~~~~

tools/test-runner.c:638:6: warning: Null pointer passed to 1st parameter
expecting 'nonnull' [core.NonNullParamChecker]
        if (chdir(home + 5) < 0) {
            ^~~~~~~~~~~~~~~

tools/test-runner.c:695:6: warning: Null pointer passed to 1st parameter
expecting 'nonnull' [core.NonNullParamChecker]
        if (chdir(home + 5) < 0) {
            ^~~~~~~~~~~~~~~

tools/test-runner.c:984:3: warning: Value stored to 'serial_fd' is never
read [deadcode.DeadStores]
                serial_fd = -1;
                ^           ~~

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>