test_harness: Report if zero tests to run.

Fixes static analysis complain about a potential malloc(0) call.

src/test/SDL_test_harness.c

@@ -443,6 +443,11 @@ int SDLTest_RunSuites(SDLTest_TestSuiteReference *testSuites[], const char *user
         }
     }
 
+    if (totalNumberOfTests == 0) {
+        SDLTest_LogError("No tests to run?");
+        return -1;
+    }
+
     /* Pre-allocate an array for tracking failed tests (potentially all test cases) */
     failedTests = (const SDLTest_TestCaseReference **)SDL_malloc(totalNumberOfTests * sizeof(SDLTest_TestCaseReference *));
     if (failedTests == NULL) {