vl: Fix an assert failure in error path

Based on the description of error_setg(), the local variable err in
qemu_maybe_daemonize() should be initialized to NULL.

Without fix, the uninitialized *errp triggers assert failure which
doesn't show much valuable information.

Before the fix:
qemu-system-x86_64: ../util/error.c:59: error_setv: Assertion `*errp == NULL' failed.

After fix:
qemu-system-x86_64: cannot create PID file: Cannot open pid file: Permission denied

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Message-Id: <20210610084741.456260-1-zhenzhong.duan@intel.com>
Cc: qemu-stable@nongnu.org
Fixes: 0546c06 ("vl: split various early command line options to a separate function", 2020-12-10)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

softmmu/vl.c

@@ -2522,7 +2522,7 @@ static void qemu_process_help_options(void)
 
 static void qemu_maybe_daemonize(const char *pid_file)
 {
-    Error *err;
+    Error *err = NULL;
 
     os_daemonize();
     rcu_disable_atfork();