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)

src/x11_window.c

@@ -1049,13 +1049,16 @@ static const char* getSelectionString(Atom selection)
 
                 if (!itemCount)
                 {
-                    if (targets[i] == XA_STRING)
+                    if (string)
                     {
-                        *selectionString = convertLatin1toUTF8(string);
-                        free(string);
+                        if (targets[i] == XA_STRING)
+                        {
+                            *selectionString = convertLatin1toUTF8(string);
+                            free(string);
+                        }
+                        else
+                            *selectionString = string;
                     }
-                    else
-                        *selectionString = string;
 
                     break;
                 }