Fix return value on error for glfwGetKeyScancode

The documentation says it should be -1, not zero.

README.md

@@ -183,6 +183,7 @@ information on what to include when reporting a bug.
  - Bugfix: `glfwMakeContextCurrent` would access TLS slot before initialization
  - Bugfix: `glfwSetGammaRamp` could emit `GLFW_INVALID_VALUE` before initialization
  - Bugfix: `glfwGetJoystickUserPointer` returned `NULL` during disconnection (#2092)
+ - Bugfix: `glfwGetKeyScancode` returned `0` on error when initialized instead of `-1`
  - [Win32] Added the `GLFW_WIN32_KEYBOARD_MENU` window hint for enabling access
            to the window menu
  - [Win32] Added a version info resource to the GLFW DLL

src/input.c

@@ -721,7 +721,7 @@ GLFWAPI int glfwGetKeyScancode(int key)
     if (key < GLFW_KEY_SPACE || key > GLFW_KEY_LAST)
     {
         _glfwInputError(GLFW_INVALID_ENUM, "Invalid key %i", key);
-        return GLFW_RELEASE;
+        return -1;
     }
 
     return _glfw.platform.getKeyScancode(key);