zero-init entrypoints in loader_add_to_dev_ext_list

[aizu-m]

ASan, teardown of an instance created with an app pfnReallocation:

SUMMARY: AddressSanitizer: SEGV loader.c:411 in free_string_list
x[8] = 0xffffffffffffffff

Chased the wild pointer back to loader_add_to_dev_ext_list. It memcpys the props sub-struct of the new element but only assigns the nested entrypoints loader_string_list when the caller hands it a non-NULL entrys. A device_extensions entry with no entrypoints array reaches it with entrys == NULL (loader.c:3097-3101, and the component-layer merge at loader.c:2653), so entrypoints keeps whatever was in the slot.

Nothing showed up until a discovered layer had 33+ device extensions. The first 32 slots come from loader_init_generic_list/loader_calloc and are zeroed, so the field read as empty. Slot 32 onwards comes from this function's own loader_instance_heap_realloc growth, and loader_realloc only zeroes the grown region on the plain realloc path, not the app pfnReallocation path (allocation.c:94-97). At vkDestroyInstance, free_string_list then walks that indeterminate list and count.

Zero entrypoints in the else branch so the appended element is fully initialised whatever allocator is in use. Regression test added under the alloc-callback suite; it poisons the grown region so the read is deterministic, and trips the SEGV without this change.

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