Gather Null specific platform selection logic

The more specific error description is now used whatever the desired
platform is, when a binary only supports the Null platform.

src/platform.c

@@ -72,6 +72,11 @@ GLFWbool _glfwSelectPlatform(int desiredID, _GLFWplatform* platform)
     // Only allow the Null platform if specifically requested
     if (desiredID == GLFW_PLATFORM_NULL)
         return _glfwConnectNull(desiredID, platform);
+    else if (count == 0)
+    {
+        _glfwInputError(GLFW_PLATFORM_UNAVAILABLE, "This binary only supports the Null platform");
+        return GLFW_FALSE;
+    }
 
     if (desiredID == GLFW_ANY_PLATFORM)
     {
@@ -86,10 +91,7 @@ GLFWbool _glfwSelectPlatform(int desiredID, _GLFWplatform* platform)
                 return GLFW_TRUE;
         }
 
-        if (count)
         _glfwInputError(GLFW_PLATFORM_UNAVAILABLE, "Failed to detect any supported platform");
-        else
-            _glfwInputError(GLFW_PLATFORM_UNAVAILABLE, "This binary only supports the Null platform");
     }
     else
     {