tests: avoid null pointer dereference

Unlikely to hit in real life, but clang noticed it.

* tests/commandtest.c (checkoutput, test4, test18): Avoid
unlink(NULL) on OOM.

tests/commandtest.c

@@ -78,6 +78,7 @@ static int checkoutput(const char *testname)
     ret = 0;
 
 cleanup:
+    if (actualname)
         unlink(actualname);
     VIR_FREE(actuallog);
     VIR_FREE(actualname);
@@ -240,6 +241,7 @@ static int test4(const void *unused ATTRIBUTE_UNUSED)
 
 cleanup:
     virCommandFree(cmd);
+    if (pidfile)
         unlink(pidfile);
     VIR_FREE(pidfile);
     return ret;
@@ -703,6 +705,7 @@ static int test18(const void *unused ATTRIBUTE_UNUSED)
 
 cleanup:
     virCommandFree(cmd);
+    if (pidfile)
         unlink(pidfile);
     VIR_FREE(pidfile);
     return ret;