"Update the image data" can dereference null "pending request" in multiple places

[awesomekling]

When implementing this algorithm for Ladybird, I had to add null checks of the "pending request", whenever calling "abort the image request" for the "pending request".

Here's how it looks in our implementation:

https://github.com/SerenityOS/serenity/blob/0f1f95da46cfa787483342fd7b982da588ea8387/Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp#L324-L326

https://github.com/SerenityOS/serenity/blob/0f1f95da46cfa787483342fd7b982da588ea8387/Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp#L384-L386

https://github.com/SerenityOS/serenity/blob/0f1f95da46cfa787483342fd7b982da588ea8387/Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp#L411-L413

https://github.com/SerenityOS/serenity/blob/0f1f95da46cfa787483342fd7b982da588ea8387/Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp#L443-L445

The spec did tell us to null check "pending request" in one place, so we're covered here:

https://github.com/SerenityOS/serenity/blob/0f1f95da46cfa787483342fd7b982da588ea8387/Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp#L454-L456

[annevk]

cc @zcorpan