Fix a SIOOBE in Unused.

This occurs from confusing source positions when "int a, b;" is desugared into "int a; int b;".

RELNOTES: N/A

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218883432

core/src/main/java/com/google/errorprone/bugpatterns/Unused.java

@@ -846,10 +846,14 @@ private static SuggestedFix replaceWithComments(
             .sorted(Comparator.<Comment>comparingInt(c -> c.getSourcePos(0)).reversed())
             .collect(toImmutableList());
     int startPos = ((JCTree) tree).getStartPosition() - startTokenization;
+    // This can happen for desugared expressions like `int a, b;`.
+    if (startPos < 0) {
+      return SuggestedFix.builder().build();
+    }
     // Delete backwards for comments which are not separated from our target by a blank line.
     for (Comment comment : comments) {
-      String stringBetweenComments =
-          source.substring(comment.getSourcePos(comment.getText().length() - 1), startPos);
+      int endOfCommentPos = comment.getSourcePos(comment.getText().length() - 1);
+      String stringBetweenComments = source.substring(endOfCommentPos, startPos);
       if (stringBetweenComments.chars().filter(c -> c == '\n').count() > 1) {
         break;
       }

core/src/test/java/com/google/errorprone/bugpatterns/UnusedTest.java

@@ -612,6 +612,21 @@ public void removal_javadocAndSingleLines() {
         .doTest(TestMode.TEXT_MATCH);
   }
 
+  @Test
+  public void unusedWithComment_interspersedComments() {
+    helper
+        .addSourceLines(
+            "UnusedWithComment.java",
+            "package unusedvars;",
+            "public class UnusedWithComment {",
+            "  private static final String foo = null, // foo",
+            "  // BUG: Diagnostic contains: Unused",
+            "      bar = null;",
+            "  public static String foo() { return foo; }",
+            "}")
+        .doTest();
+  }
+
   @Test
   public void usedInLambda() {
     helper