initialise lib_status on all failure paths in loader_scanned_icd_add

[aizu-m]

Chasing the recent lib_status work, I looked at how loader_icd_scan picks its rejection diagnostic:

loader/loader.c:4198   enum loader_layer_library_status lib_status;   // uninitialised
loader/loader.c:4200   loader_scanned_icd_add(..., &lib_status);
loader/loader.c:4205   switch (lib_status) { ... }                    // no default

loader_scanned_icd_add only writes *lib_status on some of the paths that return VK_ERROR_INCOMPATIBLE_DRIVER. Four of them do not:

:2145  NULL filename guard
:2232  missing vkCreateInstance (deprecated v0 branch)
:2241  missing vkEnumerateInstanceExtensionProperties (v0 dlsym)
:2268  missing vkEnumerateInstanceExtensionProperties (via vk_icdGetInstanceProcAddr)

A driver whose library loads but omits one of those entrypoints reaches one of these paths, and the caller then switches on an indeterminate value. The sibling paths were given a *lib_status just now, so this is a read of genuinely uninitialised stack rather than a stale-but-valid status, and the switch has no default to catch it.

Seed the out-parameter at entry so every return path leaves it defined. The specific paths still override it, so their diagnostics are unchanged.

The layer side is fine, its lib_status is a field of a calloc'd loader_layer_properties.

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[charles-lunarg]

Those failure paths should probably set lib_status (with a unique error code). I know I didn't add them in the recent PR because I didn't want to go out of scope for the PR.

Initializing lib_status is a much better long term solution regardless so appreciate the PR!

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