Fix crash when recording dependency information

Author: smarter

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -127,7 +127,14 @@ class ExtractDependencies extends Phase {
           // We can recover the fully qualified name of a classfile from
           // its path
           val classSegments = pf.givenPath.segments.takeRight(packages + 1)
-          binaryDependency(pf.file, binaryClassName(classSegments))
+          // FIXME: pf.file is null for classfiles coming from the modulepath
+          // (handled by JrtClassPath) because they cannot be represented as
+          // java.io.File, since the `binaryDependency` callback must take a
+          // java.io.File, this means that we cannot record dependencies coming
+          // from the modulepath. For now this isn't a big deal since we only
+          // support having the standard Java library on the modulepath.
+          if (pf.file != null)
+            binaryDependency(pf.file, binaryClassName(classSegments))
 
         case _ =>
           ctx.warning(s"sbt-deps: Ignoring dependency $depFile of class ${depFile.getClass}}")