[posix] Addressing coverity warning: Resource leak

The "assert" and "VerifyOrDie" were checking same condition, so
the second validation was never executed if (rval != sizeof(key)).
Additionally,"aSwapFd" was not freed on assertion.

With this commit,
1. addressed the memory leak to system resources.
2. Replaced VerifyOrDie() or assert() by VerifyOrExit()
3. handle error at exit

src/posix/platform/settings.cpp

@@ -474,18 +474,16 @@ otError PlatformSettingsDelete(otInstance *aInstance, uint16_t aKey, int aIndex,
  }
 
  rval = write(swapFd, &key, sizeof(key));
- assert(rval == sizeof(key));
- VerifyOrDie(rval == sizeof(key), OT_EXIT_FAILURE);
+ VerifyOrExit(rval == sizeof(key), error = OT_EXIT_FAILURE);
 
  rval = write(swapFd, &length, sizeof(length));
- assert(rval == sizeof(length));
- VerifyOrDie(rval == sizeof(length), OT_EXIT_FAILURE);
+ VerifyOrExit(rval == sizeof(length), error = OT_EXIT_FAILURE);
 
  swapWrite(aInstance, swapFd, length);
  }
 
 exit:
- VerifyOrDie(error != OT_ERROR_PARSE, OT_EXIT_FAILURE);
+ VerifyOrExit(error != OT_ERROR_PARSE, error = OT_EXIT_FAILURE);
 
  if (aSwapFd != nullptr)
  {
@@ -495,9 +493,10 @@ otError PlatformSettingsDelete(otInstance *aInstance, uint16_t aKey, int aIndex,
  {
  swapPersist(aInstance, swapFd);
  }
- else if (error == OT_ERROR_NOT_FOUND)
+ else if ((error == OT_ERROR_NOT_FOUND) || (error == OT_EXIT_FAILURE))
  {
  swapDiscard(aInstance, swapFd);
+ DieNow(error);
  }
 
  return error;