*: fix signedness mix-ups

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/isisd/isis_pfpacket.c b/isisd/isis_pfpacket.c
index 4bc8717..463f559 100644
--- a/isisd/isis_pfpacket.c
+++ b/isisd/isis_pfpacket.c
@@ -54,8 +54,8 @@
 u_char ALL_ISS[6] = { 0x09, 0x00, 0x2B, 0x00, 0x00, 0x05 };
 u_char ALL_ESS[6] = { 0x09, 0x00, 0x2B, 0x00, 0x00, 0x04 };
 
-static char discard_buff[8192];
-static char sock_buff[8192];
+static uint8_t discard_buff[8192];
+static uint8_t sock_buff[8192];
 
 /*
  * if level is 0 we are joining p2p multicast