Add getExtensionOrNull utility for protobuf messages

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/AnnotationAndConstantLoaderImpl.kt

@@ -90,8 +90,7 @@ class AnnotationAndConstantLoaderImpl(
             proto: ProtoBuf.Property,
             expectedType: KotlinType
     ): ConstantValue<*>? {
-        if (!proto.hasExtension(protocol.compileTimeValue)) return null
-        val value = proto.getExtension(protocol.compileTimeValue)
+        val value = proto.getExtensionOrNull(protocol.compileTimeValue) ?: return null
         return deserializer.resolveValue(expectedType, value, container.nameResolver)
     }
 }

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/ProtoBufUtil.kt

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.serialization.deserialization
+
+import org.jetbrains.kotlin.protobuf.GeneratedMessageLite
+
+fun <M : GeneratedMessageLite.ExtendableMessage<M>, T> GeneratedMessageLite.ExtendableMessage<M>.getExtensionOrNull(
+    extension: GeneratedMessageLite.GeneratedExtension<M, T>
+): T? = if (hasExtension(extension)) getExtension(extension) else null