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.

(cherry picked from commit 9c95cfb9f1)

README.md

@@ -129,6 +129,8 @@ information on what to include when reporting a bug.
    match event scancode (#1993)
  - [Win32] Bugfix: Instance-local operations used executable instance (#469,#1296,#1395)
  - [Cocoa] Bugfix: A connected Apple AirPlay would emit a useless error (#1791)
+ - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
+   `glfwGetClipboardString`
 
 
 ## Contact

src/wl_window.c

@@ -1798,20 +1798,18 @@ void _glfwPlatformSetClipboardString(const char* string)
         _glfw.wl.dataSource = NULL;
     }
 
-    if (_glfw.wl.clipboardSendString)
+    char* copy = _glfw_strdup(string);
+    if (!copy)
     {
-        free(_glfw.wl.clipboardSendString);
-        _glfw.wl.clipboardSendString = NULL;
-    }
-
-    _glfw.wl.clipboardSendString = 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);
+
+    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)