Fixed clipboard string leaks.

The clipboard string was not by the Win32 and Cocoa ports on terminate.

src/cocoa_init.m

@@ -118,6 +118,8 @@ void _glfwPlatformTerminate(void)
     [_glfw.ns.cursor release];
     _glfw.ns.cursor = nil;
 
+    free(_glfw.ns.clipboardString);
+
     _glfwTerminateJoysticks();
 
     _glfwTerminateContextAPI();

src/win32_init.c

@@ -218,6 +218,8 @@ void _glfwPlatformTerminate(void)
         _glfw.win32.classAtom = 0;
     }
 
+    free(_glfw.win32.clipboardString);
+
     _glfwTerminateContextAPI();
 
     _glfwTerminateJoysticks();

src/x11_init.c

@@ -628,14 +628,13 @@ void _glfwPlatformTerminate(void)
         _glfw.x11.cursor = (Cursor) 0;
     }
 
+    free(_glfw.x11.selection.string);
+
     _glfwTerminateJoysticks();
 
     _glfwTerminateContextAPI();
 
     terminateDisplay();
-
-    if (_glfw.x11.selection.string)
-        free(_glfw.x11.selection.string);
 }
 
 const char* _glfwPlatformGetVersionString(void)