Fix crash during error reporting

If ErrorProneScanner.scan(Tree, VisitorState) is called directly, the
TreePath may not have been initialized when it handles a crash.

RELNOTES: N/A
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118597461

core/src/main/java/com/google/errorprone/scanner/ErrorProneScanner.java

@@ -1037,10 +1037,7 @@ public Void scan(TreePath path, VisitorState state) {
     } catch (ErrorProneError e) {
       throw e;
     } catch (Throwable t) {
-      throw new ErrorProneError(
-          t,
-          (DiagnosticPosition) path.getLeaf(),
-          state.getPath().getCompilationUnit().getSourceFile());
+      throw wrapWithPosition(t, path);
     }
   }
 
@@ -1051,11 +1048,15 @@ public Void scan(Tree tree, VisitorState state) {
     } catch (ErrorProneError e) {
       throw e;
     } catch (Throwable t) {
-      throw new ErrorProneError(
-          t, (DiagnosticPosition) tree, getCurrentPath().getCompilationUnit().getSourceFile());
+      throw wrapWithPosition(t, new TreePath(getCurrentPath(), tree));
     }
   }
 
+  static ErrorProneError wrapWithPosition(Throwable t, TreePath path) {
+    return new ErrorProneError(
+        t, (DiagnosticPosition) path.getLeaf(), path.getCompilationUnit().getSourceFile());
+  }
+
   @Override
   public Map<String, SeverityLevel> severityMap() {
     return severities;