Fix for PartialFunction NPE.

Was going straight to the field and bypassing the null guard.
Closes SI-5300.

Author: paulp

src/library/scala/runtime/AbstractPartialFunction.scala

@@ -26,7 +26,7 @@ abstract class AbstractPartialFunction[-T1, +R]
   private var fallBackField: PartialFunction[T1 @uncheckedVariance, R @uncheckedVariance] = _
 
   def fallBack: PartialFunction[T1, R] = synchronized {
-    if (fallBackField == null) fallBackField = PartialFunction.empty
+    if (fallBackField eq null) fallBackField = PartialFunction.empty
     fallBackField
   }
 
@@ -38,7 +38,7 @@ abstract class AbstractPartialFunction[-T1, +R]
   override def orElse[A1 <: T1, B1 >: R](that: PartialFunction[A1, B1]) : PartialFunction[A1, B1] = {
     val result = this.clone.asInstanceOf[AbstractPartialFunction[A1, B1]]
     result.synchronized {
-      result.fallBackField = this.fallBackField orElse that
+      result.fallBackField = if (this.fallBackField eq null) that else this.fallBackField orElse that
       result
     }
   }

test/files/run/t5300.scala

@@ -0,0 +1,7 @@
+object Test {
+  val pf: PartialFunction[Any, Unit] = { case _ => () }
+  
+  def main(args: Array[String]): Unit = {
+    pf orElse pf
+  }
+}