Wayland: Fix handling of clipboard set to self

Passing any part of the result of glfwGetClipboardString to
glfwSetClipboardString would result in, at best, a use-after-free error.

README.md

@@ -310,6 +310,8 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: Some keys were reported as wrong key or `GLFW_KEY_UNKNOWN`
  - [Wayland] Bugfix: Text input did not repeat along with key repeat
  - [Wayland] Bugfix: `glfwPostEmptyEvent` sometimes had no effect (#1520,#1521)
+ - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
+   `glfwGetClipboardString`
  - [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

@@ -1664,20 +1664,18 @@ void _glfwSetClipboardStringWayland(const char* string)
         _glfw.wl.dataSource = NULL;
     }
 
-    if (_glfw.wl.clipboardSendString)
+    char* copy = _glfw_strdup(string);
+    if (!copy)
     {
-        _glfw_free(_glfw.wl.clipboardSendString);
-        _glfw.wl.clipboardSendString = NULL;
-    }
-
-    _glfw.wl.clipboardSendString = _glfw_strdup(string);
-    if (!_glfw.wl.clipboardSendString)
-    {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
-                        "Wayland: Impossible to allocate clipboard string");
+        _glfwInputError(GLFW_OUT_OF_MEMORY,
+                        "Wayland: Failed to allocate clipboard string");
         return;
     }
-    _glfw.wl.clipboardSendSize = strlen(string);
+
+    _glfw_free(_glfw.wl.clipboardSendString);
+    _glfw.wl.clipboardSendString = copy;
+
+    _glfw.wl.clipboardSendSize = strlen(_glfw.wl.clipboardSendString);
     _glfw.wl.dataSource =
         wl_data_device_manager_create_data_source(_glfw.wl.dataDeviceManager);
     if (!_glfw.wl.dataSource)