Fix short jump address calculation in x64 disassembler

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index 77599a75bb2c9fc2ec12d846deee48f60cb7c196..5c94d0c5b4bcee62d69be7aba8e44bff465a8e51 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -3156,8 +3156,16 @@ class CodeInstruction extends Observable {
     if (address == 0) {
       return;
     }
-
-    jumpTarget = instructionsByAddressOffset[address - startAddress];
+    var relativeAddress = address - startAddress;
+    if (relativeAddress < 0) {
+      Logger.root.warning('Bad address resolving jump target $relativeAddress');
+      return;
+    }
+    if (relativeAddress >= instructionsByAddressOffset.length) {
+      Logger.root.warning('Bad address resolving jump target $relativeAddress');
+      return;
+    }
+    jumpTarget = instructionsByAddressOffset[relativeAddress];
   }
 }