Fix NPE in checkLValue when selector is absent. Add test for incomplete syntax.

compiler/testData/diagnostics/tests/LValueAssignment.kt

@@ -91,6 +91,11 @@ class Test() {
         s += (a@ 2)
     }
 
+    fun testIncompleteSyntax() {
+        val s = "s"
+        <!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>++<!>s.<!SYNTAX!><!>
+    }
+
     fun testVariables() {
         var a: Int = 34
         val b: Int = 34

compiler/testData/diagnostics/tests/LValueAssignment.txt

@@ -67,6 +67,7 @@ package lvalue_assignment {
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
         internal final fun testArrays(/*0*/ a: kotlin.Array<kotlin.Int>, /*1*/ ab: lvalue_assignment.Ab): kotlin.Unit
         internal final fun testIllegalValues(): kotlin.Unit
+        internal final fun testIncompleteSyntax(): kotlin.Unit
         internal final fun testVariables(): kotlin.Unit
         internal final fun testVariables1(): kotlin.Unit
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String