fix infinite loop in terminator_EnumeratePhysicalDeviceGroups

[aizu-m]

Found by grepping loader.c for continue inside a while loop that advances its iterator at the bottom of the body. Two hits. One test driver, its vkEnumeratePhysicalDevices count query returning VK_ERROR_INITIALIZATION_FAILED:

[Vulkan Loader] ERROR:  terminator_EnumeratePhysicalDeviceGroups:  Failed during dispatch call of 'EnumeratePhysicalDevices' to ICD libtest_icd_version_2_0.dylib to get plain phys dev count.
[Vulkan Loader] ERROR:  terminator_EnumeratePhysicalDeviceGroups:  Failed during dispatch call of 'EnumeratePhysicalDevices' to ICD libtest_icd_version_2_0.dylib to get plain phys dev count.
... same line for ever, vkEnumeratePhysicalDeviceGroups never returns

The counting loop does icd_term = icd_term->next at the bottom. Both error paths continue above it, so the failing driver is queried again straight away. The trampoline holds loader_lock the whole time, so anything else taking that lock stops behind the spin. A 1.0 driver reaches the EnumeratePhysicalDevices branch because it exposes no vkEnumeratePhysicalDeviceGroups.

loader_get_fullpath is the other hit: continue on a negative snprintf return skips dir = next_dir. Folded that check into the file-exists test so the loop always advances.

Regression test added. It spins for ever before the change and passes after.

[ci-tester-lunarg]

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[ci-tester-lunarg]

Author aizu-m not on autobuild list. Waiting for curator authorization before starting CI build.

[charles-lunarg]

Very good to prevent these infinite loops. Thanks!

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