Only join the snmp thread on an unload if the thread is actually running. (issue #8810 reported by junky)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@50674 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_snmp.c

@@ -106,7 +106,7 @@ static int unload_module(void)
 	ast_verbose(VERBOSE_PREFIX_1 "Unloading [Sub]Agent Module\n");
 
 	res_snmp_dont_stop = 0;
-	return pthread_join(thread, NULL);
+	return ((thread != AST_PTHREADT_NULL) ? pthread_join(thread, NULL) : 0);
 }
 
 static int reload(void)