Add checks for some invalid values to public API

There were no checks for invalid values or asserts for all invalid NULL
pointers to glfwSetWindowIcon or glfwCreateCursor.

Fixes #1862

(cherry picked from commit 66a4882eb1)

CONTRIBUTORS.md

@@ -83,6 +83,7 @@ video tutorials.
  - Paul Holden
  - Warren Hu
  - Charles Huber
+ - Brent Huisman
  - illustris
  - InKryption
  - IntellectualKitty