audioflinger: fix NUL terminator in dumpsys

Bug: 155096545
Test: adb shell dumpsys media.audio_flinger | more
Test: then search for "Device"
Test: There should be no garbage characters on the next line.
Change-Id: I798d914cede3f45151f97178de3ceee09b4e24c0

services/audioflinger/DeviceEffectManager.cpp

@@ -144,7 +144,8 @@ void AudioFlinger::DeviceEffectManager::dump(int fd) {
         write(fd, result.string(), result.size());
     }
 
-    write(fd, "\nDevice Effects:\n", sizeof("\nDevice Effects:\n"));
+    String8 heading("\nDevice Effects:\n");
+    write(fd, heading.string(), heading.size());
     for (const auto& iter : mDeviceEffects) {
         String8 outStr;
         outStr.appendFormat("%*sEffect for device %s address %s:\n", 2, "",