Win32: Fix bad error from glfwVulkanSupported

A VK_ERROR_OUT_OF_HOST_MEMORY error would be passed on to client code on
systems that had a Vulkan loader but no ICD.

Fixes #916.

README.md

@@ -146,6 +146,8 @@ information on what to include when reporting a bug.
 - [Win32] Bugfix: Undecorated windows could not be iconified by the user (#861)
 - [Win32] Bugfix: Deadzone logic could underflow with some controllers (#910)
 - [Win32] Bugfix: Bitness test in `FindVulkan.cmake` was VS specific (#928)
+- [Win32] Bugfix: `glfwVulkanSupported` emitted an error on systems with
+                  a loader but no ICD (#916)
 - [X11] Replaced `_GLFW_HAS_XF86VM` compile-time option with dynamic loading
 - [X11] Bugfix: `glfwGetVideoMode` would segfault on Cygwin/X
 - [Linux] Bugfix: Event processing did not detect joystick disconnection (#932)

src/vulkan.c

@@ -99,9 +99,13 @@ GLFWbool _glfwInitVulkan(int mode)
     err = vkEnumerateInstanceExtensionProperties(NULL, &count, NULL);
     if (err)
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
+        // NOTE: This happens on systems with a loader but without any Vulkan ICD
-                        "Vulkan: Failed to query instance extension count: %s",
+        if (mode == _GLFW_REQUIRE_LOADER)
-                        _glfwGetVulkanResultString(err));
+        {
+            _glfwInputError(GLFW_PLATFORM_ERROR,
+                            "Vulkan: Failed to query instance extension count: %s",
+                            _glfwGetVulkanResultString(err));
+        }
 
         _glfwTerminateVulkan();
         return GLFW_FALSE;