Fix NPE in akka-http and pekko-http integrations (#9019)

Also fix a related issue where spans were not being finished while polling through leftover scopes.

Author: mcculls

dd-java-agent/instrumentation/akka-http/akka-http-10.0/src/main/java/datadog/trace/instrumentation/akkahttp/DatadogServerRequestResponseFlowWrapper.java

@@ -168,16 +168,16 @@ public void onUpstreamFinish() throws Exception {
               @Override
               public void onUpstreamFailure(final Throwable ex) throws Exception {
                 ContextScope scope = scopes.poll();
-                AgentSpan span = AgentSpan.fromContext(scope.context());
                 if (scope != null) {
                   // Mark the span as failed
+                  AgentSpan span = AgentSpan.fromContext(scope.context());
                   DatadogWrapperHelper.finishSpan(span, ex);
                 }
                 // We will not receive any more responses from the user code, so clean up any
                 // remaining spans
                 scope = scopes.poll();
                 while (scope != null) {
-                  span.finish();
+                  AgentSpan.fromContext(scope.context()).finish();
                   scope = scopes.poll();
                 }
                 fail(responseOutlet, ex);

dd-java-agent/instrumentation/pekko-http-1.0/src/main/java/datadog/trace/instrumentation/pekkohttp/DatadogServerRequestResponseFlowWrapper.java

@@ -141,16 +141,16 @@ public void onUpstreamFinish() throws Exception {
               @Override
               public void onUpstreamFailure(final Throwable ex) throws Exception {
                 ContextScope scope = scopes.poll();
-                AgentSpan span = AgentSpan.fromContext(scope.context());
                 if (scope != null) {
                   // Mark the span as failed
+                  AgentSpan span = AgentSpan.fromContext(scope.context());
                   DatadogWrapperHelper.finishSpan(span, ex);
                 }
                 // We will not receive any more responses from the user code, so clean up any
                 // remaining spans
                 scope = scopes.poll();
                 while (scope != null) {
-                  span.finish();
+                  AgentSpan.fromContext(scope.context()).finish();
                   scope = scopes.poll();
                 }
                 fail(responseOutlet, ex);