fix error when frames is nil and add []

Author: vpellan

lib/datadog/appsec/stack_trace.rb

@@ -22,7 +22,7 @@ def initialize(frames)
                        end
           bottom_frames = max_depth - top_frames
           cropped_frames = []
-          frames.each_with_index do |frame, index|
+          frames && frames.each_with_index do |frame, index|
             next if index >= top_frames && index < frames.size - bottom_frames
 
             # ASCII-8BIT is encoded as byte array and backend cannot decode it properly
@@ -47,6 +47,10 @@ def each(&block)
           @frames.each(&block)
         end
 
+        def [](index)
+          @frames[index]
+        end
+
         def to_msgpack(packer = nil)
           packer ||= MessagePack::Packer.new