Define default pidfile when creating pid

We want pidfile to be NULL on startup so we can detect if the user
set an explicit value versus only using the default value.

Closes redis#1967
Fixes redis#2076

Author: mattsta

src/redis.c

@@ -1433,7 +1433,7 @@ void initServerConfig(void) {
     server.aof_flush_postponed_start = 0;
     server.aof_rewrite_incremental_fsync = REDIS_DEFAULT_AOF_REWRITE_INCREMENTAL_FSYNC;
     server.aof_load_truncated = REDIS_DEFAULT_AOF_LOAD_TRUNCATED;
-    server.pidfile = zstrdup(REDIS_DEFAULT_PID_FILE);
+    server.pidfile = NULL;
     server.rdb_filename = zstrdup(REDIS_DEFAULT_RDB_FILENAME);
     server.aof_filename = zstrdup(REDIS_DEFAULT_AOF_FILENAME);
     server.requirepass = NULL;
@@ -3393,6 +3393,10 @@ void linuxMemoryWarnings(void) {
 #endif /* __linux__ */
 
 void createPidFile(void) {
+    /* If pidfile requested, but no pidfile defined, use
+     * default pidfile path */
+    if (!server.pidfile) server.pidfile = zstrdup(REDIS_DEFAULT_PID_FILE);
+
     /* Try to write the pid file in a best-effort way. */
     FILE *fp = fopen(server.pidfile,"w");
     if (fp) {