Fix #7742: Ensure the indentWidth is never null

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -1359,7 +1359,8 @@ object Scanners {
 
   case class InBraces(var width: IndentWidth | Null, outer: Region)
   extends IndentSignificantRegion {
-    override def indentWidth = width
+    override def indentWidth =
+      if width == null then IndentWidth.Zero else width
   }
 
   /** A class describing an indentation region.

tests/neg/i7742.scala

@@ -0,0 +1,3 @@
+object A {
+    for // error // error
+}

tests/pos/i7741.scala

@@ -0,0 +1,9 @@
+class A1 {
+    @native private def a: Unit
+}
+trait A2 {
+    erased def i(a: Int): Int
+}
+trait A3 {
+    erased val a: Int
+}