Fix sdltest_randomAsciiStringWithMaximumLength() where it requests a string a size 0

seen with: ./testautomation --seed MILAFAP2AKVP3V4G --filter sdltest_randomAsciiStringWithMaximumLength

src/test/SDL_test_fuzzer.c

@@ -470,7 +470,9 @@ SDLTest_RandomAsciiStringWithMaximumLength(int maxLength)
     }
 
     size = (SDLTest_RandomUint32() % (maxLength + 1));
-
+    if (size == 0) {
+        size = 1;
+    }
     return SDLTest_RandomAsciiStringOfSize(size);
 }