refresh_token error when i'am pass token failed

hi there,

iam trying to create testing file with a middleware (gin jwt), so when i'am access the page /refresh_token and then iam pass the wrong value of prev token, i got a panic "invalid memory address or nil pointer dereference"

i check your code and i think you must validate a token
on line: 415 : token, _ := mw.parseToken(c)

because your function parseToken() retrun nil when error

maybe it's snipped can help.

added online 416:

if token == nil {
mw.unauthorized(c, http.StatusUnauthorized, mw.HTTPStatusMessageFunc(ErrExpiredToken, c))
return
}

Start at the refresh_token middleware path around the reported line where parseToken(c) is called, and reproduce the request with an invalid previous token. Trace the nil result and confirm that the completed behavior returns an unauthorized response instead of panicking.

Written by the indexing model from the issue text.