[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, addressed the memory leak to system resources.

src/posix/platform/settings.cpp

@@ -474,12 +474,18 @@ 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);
+ if(rval != sizeof(key))
+ {
+ swapDiscard(aInstance, swapFd);
+ VerifyOrDie(rval == sizeof(key), OT_EXIT_FAILURE);
+ }
 
  rval = write(swapFd, &length, sizeof(length));
- assert(rval == sizeof(length));
- VerifyOrDie(rval == sizeof(length), OT_EXIT_FAILURE);
+ if(rval != sizeof(length))
+ {
+ swapDiscard(aInstance, swapFd);
+ VerifyOrDie(rval == sizeof(length), OT_EXIT_FAILURE);
+ }
 
  swapWrite(aInstance, swapFd, length);
  }