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)

src/input.c

@@ -743,9 +743,16 @@ 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 = calloc(1, sizeof(_GLFWcursor));
     cursor->next = _glfw.cursorListHead;
     _glfw.cursorListHead = cursor;

src/window.c

@@ -504,12 +504,33 @@ GLFWAPI void glfwSetWindowTitle(GLFWwindow* handle, const char* title)
 GLFWAPI void glfwSetWindowIcon(GLFWwindow* handle,
                                int count, const GLFWimage* images)
 {
+    int i;
     _GLFWwindow* window = (_GLFWwindow*) handle;
+
     assert(window != NULL);
     assert(count >= 0);
     assert(count == 0 || images != NULL);
 
     _GLFW_REQUIRE_INIT();
+
+    if (count < 0)
+    {
+        _glfwInputError(GLFW_INVALID_VALUE, "Invalid image count for window icon");
+        return;
+    }
+
+    for (i = 0; i < count; i++)
+    {
+        assert(images[i].pixels != NULL);
+
+        if (images[i].width <= 0 || images[i].height <= 0)
+        {
+            _glfwInputError(GLFW_INVALID_VALUE,
+                            "Invalid image dimensions for window icon");
+            return;
+        }
+    }
+
     _glfwPlatformSetWindowIcon(window, count, images);
 }