Cocoa: Fix work area retrieval on missing screen

Closes #1322.

src/cocoa_monitor.m

@@ -413,13 +413,7 @@ void _glfwPlatformGetMonitorWorkarea(_GLFWmonitor* monitor, int* xpos, int* ypos
     NSScreen *resultScreen;
     for (NSScreen *screen in [NSScreen screens]) {
         if ([[[screen deviceDescription] valueForKey:@"NSScreenNumber"] intValue] == monitor->ns.displayID) {
-            resultScreen = screen;
+            NSRect frameRect = [screen visibleFrame];
-            break;
-        }
-    }
-
-    NSRect frameRect = [[NSScreen resultScreen] visibleFrame];
-
             if (xpos)
                 *xpos = NSMinX(frameRect);
             if (ypos)
@@ -428,6 +422,9 @@ void _glfwPlatformGetMonitorWorkarea(_GLFWmonitor* monitor, int* xpos, int* ypos
                 *width = NSWidth(frameRect);
             if (height)
                 *height = NSHeight(frameRect);
+            break;
+        }
+    }
 }
 
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* count)