Wayland: Fix double free on data source error

If data source creation fails, the string containing the data for it
would be freed a second time during termination.

README.md

@@ -312,6 +312,7 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: `glfwPostEmptyEvent` sometimes had no effect (#1520,#1521)
  - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
    `glfwGetClipboardString`
+ - [Wayland] Bugfix: Data source creation error would cause double free at termination
  - [POSIX] Removed use of deprecated function `gettimeofday`
  - [POSIX] Bugfix: `CLOCK_MONOTONIC` was not correctly tested for or enabled
  - [WGL] Disabled the DWM swap interval hack for Windows 8 and later (#1072)

src/wl_window.c

@@ -1683,6 +1683,7 @@ void _glfwSetClipboardStringWayland(const char* string)
         _glfwInputError(GLFW_PLATFORM_ERROR,
                         "Wayland: Impossible to create clipboard source");
         _glfw_free(_glfw.wl.clipboardSendString);
+        _glfw.wl.clipboardSendString = NULL;
         return;
     }
     wl_data_source_add_listener(_glfw.wl.dataSource,