Fix heap overflow when reading config.

Closes oss-fuzz #57002.

src/conf.c

@@ -1842,15 +1842,16 @@ static int config__read_file_core(struct mosquitto__config *config, bool reload,
 						cr->log_dest |= MQTT3_LOG_ANDROID;
 #endif
 					}else if(!strcmp(token, "file")){
-						cr->log_dest |= MQTT3_LOG_FILE;
 						if(config->log_fptr || config->log_file){
 							log__printf(NULL, MOSQ_LOG_ERR, "Error: Duplicate \"log_dest file\" value.");
 							return MOSQ_ERR_INVAL;
 						}
 						/* Get remaining string. */
-						token = &token[strlen(token)+1];
-						while(token[0] == ' ' || token[0] == '\t'){
-							token++;
+						token = saveptr;
+						if(token && token[0]){
+							while(token[0] == ' ' || token[0] == '\t'){
+								token++;
+							}
 						}
 						if(token[0]){
 							config->log_file = mosquitto__strdup(token);
@@ -1862,6 +1863,7 @@ static int config__read_file_core(struct mosquitto__config *config, bool reload,
 							log__printf(NULL, MOSQ_LOG_ERR, "Error: Empty \"log_dest file\" value in configuration.");
 							return MOSQ_ERR_INVAL;
 						}
+						cr->log_dest |= MQTT3_LOG_FILE;
 					}else{
 						log__printf(NULL, MOSQ_LOG_ERR, "Error: Invalid 'log_dest' value (%s).", token);
 						return MOSQ_ERR_INVAL;