null-robustness for presentation test

ran into NPEs while running the test suite using the virtpatmat compiler,
decided it might happen to others
thus undertook to shield said others from sad NPEs

no review

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@26007 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/compiler/scala/tools/nsc/interactive/tests/core/CoreTestDefs.scala

@@ -81,7 +81,11 @@ private[tests] trait CoreTestDefs
         else {
           reporter.println("\naskHyperlinkPos for `" + tree.symbol.name + "` at " + format(pos) + " " + pos.source.file.name)
           val r = new Response[Position]
-          val sourceFile = sourceFiles.find(tree.symbol.sourceFile.path == _.path) match {
+          // `tree.symbol.sourceFile` was discovered to be null when testing -Yvirtpatmat on the akka presentation test, where a position had shifted to point to `Int`
+          // askHyperlinkPos for `Int` at (73,19) pi.scala --> class Int in package scala has null sourceFile!
+          val treePath = if (tree.symbol.sourceFile ne null) tree.symbol.sourceFile.path else null
+          val treeName = if (tree.symbol.sourceFile ne null) tree.symbol.sourceFile.name else null
+          val sourceFile = sourceFiles.find(_.path == treePath) match {
             case Some(source) =>
               compiler.askLinkPos(tree.symbol, source, r)
               r.get match {
@@ -93,7 +97,7 @@ private[tests] trait CoreTestDefs
                   ex.printStackTrace()
               }
             case None =>
-              reporter.println("[error] could not locate sourcefile `" + tree.symbol.sourceFile.name + "`." +
+              reporter.println("[error] could not locate sourcefile `" + treeName + "`." +
                 "Hint: Does the looked up definition come form a binary?")
           }
         }