buffer: fix incorrect unlock of the buffer mutex (for deferred callbacks)

buffer: fix incorrect unlock of the buffer mutex (for deferred callbacks)

TSAN reports:
  WARNING: ThreadSanitizer: unlock of an unlocked mutex (or by a wrong thread) (pid=17111)
      #0 pthread_mutex_unlock /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3621 (libtsan.so.0+0x00000003b71c)
      #1 evbuffer_add <null> (libevent_core-2.2.so.1+0x00000000ddb6)
      ...

    Mutex M392 (0x7b0c00000f00) created at:
      #0 pthread_mutex_init /build/gcc/src/gcc/libsanitizer/tsan/tsan_interceptors.cc:1117 (libtsan.so.0+0x0000000291af)
      #1 <null> <null> (libevent_pthreads-2.2.so.1+0x000000000d46)
      ...

$ addr2line -e /lib/libevent_core-2.2.so.1 0x00000000ddb6
/src/libevent/buffer.c:1815 (discriminator 1)

Introduced-in: ae2b84b2575be93d0aebba5c0b78453836f89f3c ("Replace
deferred_cbs with event_callback-based implementation.")