Fix uninitialized memory use

Author: Beyley

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -11619,7 +11619,7 @@ static Uint8 VULKAN_INTERNAL_GetXRPhysicalDevice(VulkanRenderer *renderer)
     XrResult xrResult;
     VulkanExtensions physicalDeviceExtension;
     Uint32 queueFamilyIndex;
-    Uint8 deviceRank;
+    Uint8 deviceRank = 0;
 
     PFN_xrGetVulkanGraphicsDevice2KHR xrGetVulkanGraphicsDevice2KHR;
     if((xrResult = xrGetInstanceProcAddr(renderer->xrInstance, "xrGetVulkanGraphicsDevice2KHR", (PFN_xrVoidFunction*)&xrGetVulkanGraphicsDevice2KHR)) != XR_SUCCESS) {