Don't suggest making interface member final

(which was possible in leaking this inspection)
So #KT-14443 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/LeakingThisInspection.kt

@@ -29,6 +29,7 @@ import org.jetbrains.kotlin.idea.caches.resolve.analyzeFully
 import org.jetbrains.kotlin.idea.quickfix.AddModifierFix
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.psi.psiUtil.containingClassOrObject
 import org.jetbrains.kotlin.resolve.BindingContext.LEAKING_THIS
 import org.jetbrains.kotlin.resolve.DescriptorToSourceUtils
 
@@ -88,6 +89,7 @@ class LeakingThisInspection : AbstractKotlinInspection() {
             declaration ?: return null
             val useScope = declaration.useScope
             if (DefinitionsScopedSearch.search(declaration, useScope).findFirst() != null) return null
+            if ((declaration.containingClassOrObject as? KtClass)?.isInterface() ?: false) return null
             return IntentionWrapper(AddModifierFix(declaration, KtTokens.FINAL_KEYWORD), declaration.containingFile)
         }
     }

idea/testData/inspectionsLocal/leakingThis/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.LeakingThisInspection

idea/testData/inspectionsLocal/leakingThis/noOpenForInterface.kt

@@ -0,0 +1,14 @@
+// PROBLEM: Calling non-final function foo in constructor
+
+interface My {
+    fun foo() {}
+}
+
+open class Your : My {
+
+    init {
+        // exactly one fix action should be here (make 'Your' final)
+        // making 'foo' final will provoke an error
+        <caret>foo()
+    }
+}

idea/testData/inspectionsLocal/leakingThis/noOpenForInterface.kt.after

@@ -0,0 +1,14 @@
+// PROBLEM: Calling non-final function foo in constructor
+
+interface My {
+    fun foo() {}
+}
+
+class Your : My {
+
+    init {
+        // exactly one fix action should be here (make 'Your' final)
+        // making 'foo' final will provoke an error
+        <caret>foo()
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -63,6 +63,21 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
         }
     }
 
+    @TestMetadata("idea/testData/inspectionsLocal/leakingThis")
+    @TestDataPath("$PROJECT_ROOT")
+    @RunWith(JUnit3RunnerWithInners.class)
+    public static class LeakingThis extends AbstractLocalInspectionTest {
+        public void testAllFilesPresentInLeakingThis() throws Exception {
+            KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/testData/inspectionsLocal/leakingThis"), Pattern.compile("^([\\w\\-_]+)\\.kt$"), TargetBackend.ANY, true);
+        }
+
+        @TestMetadata("noOpenForInterface.kt")
+        public void testNoOpenForInterface() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/leakingThis/noOpenForInterface.kt");
+            doTest(fileName);
+        }
+    }
+
     @TestMetadata("idea/testData/inspectionsLocal/moveSuspiciousCallableReferenceIntoParentheses")
     @TestDataPath("$PROJECT_ROOT")
     @RunWith(JUnit3RunnerWithInners.class)