[posix] addressing coverity warning: resource leak (openthread#9472)

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

@@ -436,7 +436,7 @@ otError PlatformSettingsDelete(otInstance *aInstance, uint16_t aKey, int aIndex,
 
     assert(swapFd != -1);
     assert(offset == 0);
-    VerifyOrExit(offset == 0 && size >= 0, error = OT_ERROR_PARSE);
+    VerifyOrExit(offset == 0 && size >= 0, error = OT_ERROR_FAILED);
 
     while (offset < size)
     {
@@ -445,25 +445,25 @@ otError PlatformSettingsDelete(otInstance *aInstance, uint16_t aKey, int aIndex,
         ssize_t  rval;
 
         rval = read(sSettingsFd, &key, sizeof(key));
-        VerifyOrExit(rval == sizeof(key), error = OT_ERROR_PARSE);
+        VerifyOrExit(rval == sizeof(key), error = OT_ERROR_FAILED);
 
         rval = read(sSettingsFd, &length, sizeof(length));
-        VerifyOrExit(rval == sizeof(length), error = OT_ERROR_PARSE);
+        VerifyOrExit(rval == sizeof(length), error = OT_ERROR_FAILED);
 
         offset += sizeof(key) + sizeof(length) + length;
 
         if (aKey == key)
         {
             if (aIndex == 0)
             {
-                VerifyOrExit(offset == lseek(sSettingsFd, length, SEEK_CUR), error = OT_ERROR_PARSE);
+                VerifyOrExit(offset == lseek(sSettingsFd, length, SEEK_CUR), error = OT_ERROR_FAILED);
                 swapWrite(aInstance, swapFd, static_cast<uint16_t>(size - offset));
                 error = OT_ERROR_NONE;
                 break;
             }
             else if (aIndex == -1)
             {
-                VerifyOrExit(offset == lseek(sSettingsFd, length, SEEK_CUR), error = OT_ERROR_PARSE);
+                VerifyOrExit(offset == lseek(sSettingsFd, length, SEEK_CUR), error = OT_ERROR_FAILED);
                 error = OT_ERROR_NONE;
                 continue;
             }
@@ -474,19 +474,15 @@ 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_ERROR_FAILED);
 
         rval = write(swapFd, &length, sizeof(length));
-        assert(rval == sizeof(length));
-        VerifyOrDie(rval == sizeof(length), OT_EXIT_FAILURE);
+        VerifyOrExit(rval == sizeof(length), error = OT_ERROR_FAILED);
 
         swapWrite(aInstance, swapFd, length);
     }
 
 exit:
-    VerifyOrDie(error != OT_ERROR_PARSE, OT_EXIT_FAILURE);
-
     if (aSwapFd != nullptr)
     {
         *aSwapFd = swapFd;
@@ -499,6 +495,11 @@ otError PlatformSettingsDelete(otInstance *aInstance, uint16_t aKey, int aIndex,
     {
         swapDiscard(aInstance, swapFd);
     }
+    else if (error == OT_ERROR_FAILED)
+    {
+        swapDiscard(aInstance, swapFd);
+        DieNow(error);
+    }
 
     return error;
 }