Fix emission of empty exception handlers in Dotty

Detection of any instructions were emitted wasn't robust enough,
as it assumes that any ASM node would result in some byte code instruction being emmited.
This is not true for LineNumber nodes and exploded with
`ClassFormatError: Illegal exception Table range`

Author: DarkDimius

src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala

@@ -453,7 +453,13 @@ trait BCodeSkelBuilder extends BCodeHelpers {
       if (!skip) { mnode visitLabel lbl }
     }
     def isAtProgramPoint(lbl: asm.Label): Boolean = {
-      (lastInsn match { case labnode: asm.tree.LabelNode => (labnode.getLabel == lbl); case _ => false } )
+      def getNonLineNumberNode(a: asm.tree.AbstractInsnNode): asm.tree.AbstractInsnNode  = a match {
+        case a: asm.tree.LineNumberNode => getNonLineNumberNode(a.getPrevious) // line numbers aren't part of code itself
+        case _ => a
+      }
+      (getNonLineNumberNode(lastInsn) match {
+        case labnode: asm.tree.LabelNode => (labnode.getLabel == lbl);
+        case _ => false } )
     }
     def lineNumber(tree: Tree) {
       if (!emitLines || !tree.pos.isDefined) return;