Vk: well OF COURSE I made an error when using std::lower_bound()!

Goddammit, C++, why the hell all APIs have to be so error prone. You
didn't learn from C's mistakes at all, did you.

src/Magnum/Vk/ExtensionProperties.cpp

@@ -145,7 +145,7 @@ UnsignedInt ExtensionProperties::revision(const Containers::StringView extension
     /* Thanks, C++, for forcing me to have a larger bug surface instead of
        providing a library helper to find the damn thing. */
     auto found = std::lower_bound(_names.begin(), _names.end(), extension);
-    if(*found != extension) return 0;
+    if(found == _names.end() || *found != extension) return 0;
 
     /* The view target is contents of the VkExtensionProperties structure,
        the revision is stored nearby */

src/Magnum/Vk/Test/ExtensionPropertiesVkTest.cpp

@@ -208,6 +208,9 @@ void ExtensionPropertiesVkTest::namedRevision() {
 
     /* Unknown extensions return 0 */
     CORRADE_COMPARE(properties.revision("VK_this_doesnt_exist"), 0);
+    /* Verify that we don't dereference garbage when std::lower_bound() returns
+       `last` */
+    CORRADE_COMPARE(properties.revision("ZZZZZ"), 0);
 
     /* Verify the overloads that take our extension wrappers work as well */
     CORRADE_COMPARE_AS(properties.revision<Extensions::KHR::get_physical_device_properties2>(), 0,

src/Magnum/Vk/Test/LayerPropertiesVkTest.cpp

@@ -125,6 +125,9 @@ void LayerPropertiesVkTest::isSupported() {
     LayerProperties properties = enumerateLayerProperties();
 
     CORRADE_VERIFY(!properties.isSupported("this layer doesn't exist"));
+    /* Verify that we don't dereference garbage when std::lower_bound() returns
+       `last` */
+    CORRADE_VERIFY(!properties.isSupported("ZZZZZ"));
 
     if(!properties.count())
         CORRADE_SKIP("The driver reported no instance layers, can't fully test.");