Fix incorrect assertion in FIR DFA

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirDataFlowAnalyzer.kt

@@ -561,8 +561,7 @@ class FirDataFlowAnalyzer(private val components: FirAbstractBodyResolveTransfor
          *      x.length
          *   }
          */
-        variableStorage[initializer]?.let { initializerVariable ->
-            assert(initializerVariable.isSynthetic())
+        variableStorage[initializer]?.takeIf { it.isSynthetic() }?.let { initializerVariable ->
             val realVariable = getOrCreateRealVariable(variable)
             requireNotNull(realVariable)
             logicSystem.changeVariableForConditionFlow(node.flow, initializerVariable, realVariable)

compiler/fir/resolve/testData/resolve/stdlib/anonymousInDelegate.txt

@@ -0,0 +1,23 @@
+FILE: anonymousInDelegate.kt
+    public abstract interface Foo : R|kotlin/Any| {
+        public abstract fun bar(): R|kotlin/Int|
+
+    }
+    public final val x: R|kotlin/Int|by R|kotlin/lazy|<R|kotlin/Int|>(<L> = lazy@fun <anonymous>(): R|kotlin/Int| {
+        lval foo: R|anonymous| = object : R|Foo| {
+            private constructor(): R|kotlin/Any| {
+                super<R|kotlin/Any|>()
+            }
+
+            public final override fun bar(): R|kotlin/Int| {
+                ^bar Int(42)
+            }
+
+        }
+
+        R|<local>/foo|.R|/bar|()
+    }
+    )
+        public get(): R|kotlin/Int| {
+            ^ D|/x|.R|kotlin/getValue|<R|kotlin/Int|>(Null(null), ::R|/x|)
+        }