skip layers missing api_version or implementation_version

[aizu-m]

A layer manifest that omits api_version is logged as skipped, but still shows up:

[Vulkan Loader] WARNING: ... didn't find required layer value "api_version" in manifest JSON file, skipping this layer
// vkEnumerateInstanceLayerProperties then still lists the layer

loader_read_layer_json seeds result to VK_SUCCESS and every required-field check sets it back to VK_ERROR_INITIALIZATION_FAILED before jumping to out: (name, type, the api_version variant check, description, disable_environment, library_arch). The out: epilogue only appends the layer when result stayed VK_SUCCESS. The two missing-field checks for api_version and implementation_version jump to out: without setting the failure result, so the layer is appended anyway.

Both checks sit above the library_path parsing, so the appended layer never gets a lib_name. Once such a layer is activated the loader calls loader_platform_open_library(NULL). dlopen(NULL) returns the host program handle, and loader_open_layer_file then logs the NULL lib_name through a %s conversion. Layer manifests are untrusted input, so a manifest dropping one key is enough to reach this.

Set the failure result in both checks to match the neighbours. Added a regression test covering both fields.

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