X11: Fix segfault on malformed INCR response

The code assumed that at least some data would be received via the INCR
mechanism and that, as a result, the string buffer would be allocated.

Bug found by Clang static analysis.

(cherry picked from commit 23e6e8e4b7)

README.md

@@ -134,6 +134,7 @@ information on what to include when reporting a bug.
  - [Cocoa] Bugfix: A connected Apple AirPlay would emit a useless error (#1791)
  - [Cocoa] Bugfix: The EGL and OSMesa libraries were not unloaded on termination
  - [X11] Bugfix: The OSMesa libray was not unloaded on termination
+ - [X11] Bugfix: A malformed response during selection transfer could cause a segfault
  - [Wayland] Added support for file path drop events (#2040)
  - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
    `glfwGetClipboardString`

src/x11_window.c

@@ -1048,6 +1048,8 @@ static const char* getSelectionString(Atom selection)
                 }
 
                 if (!itemCount)
+                {
+                    if (string)
                     {
                         if (targets[i] == XA_STRING)
                         {
@@ -1056,6 +1058,7 @@ static const char* getSelectionString(Atom selection)
                         }
                         else
                             *selectionString = string;
+                    }
 
                     break;
                 }