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

src/input.c

@@ -764,9 +764,16 @@ GLFWAPI GLFWcursor* glfwCreateCursor(const GLFWimage* image, int xhot, int yhot)
     _GLFWcursor* cursor;
 
     assert(image != NULL);
+    assert(image->pixels != NULL);
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
+    if (image->width <= 0 || image->height <= 0)
+    {
+        _glfwInputError(GLFW_INVALID_VALUE, "Invalid image dimensions for cursor");
+        return NULL;
+    }
+
     cursor = _glfw_calloc(1, sizeof(_GLFWcursor));
     cursor->next = _glfw.cursorListHead;
     _glfw.cursorListHead = cursor;