KT-418: test for case when safe call on non-null value returns null

1?.somethingReturnNull()

must still yield null

compiler/testData/checkerWithErrorTypes/quick/SafeCallNonNullReceiverReturnNull.jet

@@ -0,0 +1,6 @@
+fun Int.gg() = null
+
+fun ff() {
+    val a: Int = 1
+    val b: Int = <!TYPE_MISMATCH!>a<!UNNECESSARY_SAFE_CALL!>?.<!>gg()<!>
+}