nss.c: Fixed compilation warnings (Take Two)

nss.c:702: warning: pointer targets in passing argument 3 of
'Curl_extract_certinfo' differ in signedness

nss.c:702: warning: pointer targets in passing argument 4 of
'Curl_extract_certinfo' differ in signedness

Made sure the cast was correctly "unsigned char *" to "char *" and not
"unsigned char *" to "unsigned char *".