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.

README.md

@@ -290,6 +290,7 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: Dynamic loading on NetBSD failed due to soname differences
  - [X11] Bugfix: Left shift of int constant relied on undefined behavior (#1951)
  - [X11] Bugfix: The OSMesa libray was not unloaded on termination
+ - [X11] Bugfix: A malformed response during selection transfer could cause a segfault
  - [Wayland] Added dynamic loading of all Wayland libraries
  - [Wayland] Added support for key names via xkbcommon
  - [Wayland] Added support for file path drop events (#2040)

src/x11_window.c

@@ -984,6 +984,8 @@ static const char* getSelectionString(Atom selection)
                 }
 
                 if (!itemCount)
+                {
+                    if (string)
                     {
                         if (targets[i] == XA_STRING)
                         {
@@ -992,6 +994,7 @@ static const char* getSelectionString(Atom selection)
                         }
                         else
                             *selectionString = string;
+                    }
 
                     break;
                 }