[trel] fix crash (openthread#10894)

Started from openthread#10872, platform trel stays un-initialized if no TREL URL is
passed in. `otPlatTrelEnable` asserted for `sInitialized`, and caused
crashes when `otPlatTrelEnable()` is called.

This commit changes the assertions to VerifyOrExit() to allow
otPlatTrelEnable calls when TREL is not initialized.

src/posix/platform/trel.cpp

@@ -474,9 +474,7 @@ void otPlatTrelEnable(otInstance *aInstance, uint16_t *aUdpPort)
 
     VerifyOrExit(!IsSystemDryRun());
 
-    assert(sInitialized);
-
-    VerifyOrExit(!sEnabled);
+    VerifyOrExit(sInitialized && !sEnabled);
 
     PrepareSocket(*aUdpPort);
     trelDnssdStartBrowse();
@@ -493,8 +491,7 @@ void otPlatTrelDisable(otInstance *aInstance)
 
     VerifyOrExit(!IsSystemDryRun());
 
-    assert(sInitialized);
-    VerifyOrExit(sEnabled);
+    VerifyOrExit(sInitialized && sEnabled);
 
     close(sSocket);
     sSocket = -1;
@@ -541,6 +538,8 @@ void otPlatTrelRegisterService(otInstance *aInstance, uint16_t aPort, const uint
     OT_UNUSED_VARIABLE(aInstance);
     VerifyOrExit(!IsSystemDryRun());
 
+    VerifyOrExit(sEnabled);
+
     trelDnssdRegisterService(aPort, aTxtData, aTxtLength);
 
 exit: