BLE Host - Free mbufs on exec-write-failure
BLE Host - Free mbufs on exec-write-failure

The previous fix didn't fix an mbuf leak.  It added one!  The real bug
was that an exec-write-response (success) gets sent when a write fails.
This commit fixes the mbuf leak that was introduced.

X-Original-Commit: 36de005b3dc0b18aeace6d58d9a7f167294fba2a

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble