SI-9170 Fix resident compilation / specialization NPE

The resident compiler does its best to clean the decks at
the conclusion of a compilation batch.

One part of this is as follows: if the run was erroneous,
reset the info of top level symbols defined in this run
to the initial state, that is, to a `SourceFileLoader`.

However, if the errors came late in the compilation pipeline,
the map from symbols to the source files includes the results
of the specialization transformation, which ends up with
mappings like `Function1$sp... -> null`.

This results in a `NullPointerException` on subsequent runs.

This commits filters out null source files during the reset
process.

1 files changed, 4 insertions, 0 deletions

diff --git a/test/files/res/t9170/A.scala b/test/files/res/t9170/A.scala
new file mode 100644
index 0000000000..239df89679
--- /dev/null
+++ b/test/files/res/t9170/A.scala
@@ -0,0 +1,4 @@
+object Y {
+  def f[A](a: =>  A) = 1
+  def f[A](a: => Either[Exception, A]) = 2
+}