snapshot/mac: MachOImageAnnotationsReader test shouldn’t crash with a nullptr ProcessReader::Module

snapshot/mac/process_reader.h

Index: snapshot/mac/process_reader.h
diff --git a/snapshot/mac/process_reader.h b/snapshot/mac/process_reader.h
index 0227adecd4b998d2a4f518614330dd7fd6361fec..a781667f2829d0042d22bf6ad91f665bbef1dc43 100644
--- a/snapshot/mac/process_reader.h
+++ b/snapshot/mac/process_reader.h
@@ -84,6 +84,9 @@ class ProcessReader {
     //!
     //! The lifetime of this MachOImageReader is scoped to the lifetime of the
     //! ProcessReader that created it.
+    //!
+    //! This field may be `nullptr` if a reader could not be created for the
+    //! module.
     const MachOImageReader* reader;
 
     //! \brief The module’s timestamp.