bgpd rework extract_prefix

[Claudio Jeker]

This diff cleans up extract_prefix(). The main point is to abort before
writing anything.

PREFIX_SIZE() includes the length byte so exclude that again.
I think overall the function is more readable now.
-- 
:wq Claudio

Index: util.c
===================================================================
RCS file: /cvs/src/usr.sbin/bgpd/util.c,v
retrieving revision 1.74
diff -u -p -r1.74 util.c
--- util.c      4 Jan 2023 14:33:30 -0000       1.74
+++ util.c      30 Mar 2023 13:09:52 -0000
@@ -501,19 +501,18 @@ extract_prefix(u_char *p, uint16_t len, 
        static u_char    addrmask[] = {
            0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff };
        u_char          *a = va;
-       int              i;
-       uint16_t         plen = 0;
+       int              plen;
 
-       for (i = 0; pfxlen && i < max; i++) {
-               if (len <= plen)
-                       return (-1);
+       plen = PREFIX_SIZE(pfxlen) - 1;
+       if (len < plen || max < plen)
+               return -1;
+
+       while (pfxlen > 0) {
                if (pfxlen < 8) {
-                       a[i] = *p++ & addrmask[pfxlen];
-                       plen++;
+                       *a++ = *p++ & addrmask[pfxlen];
                        break;
                } else {
-                       a[i] = *p++;
-                       plen++;
+                       *a++ = *p++;
                        pfxlen -= 8;
                }
        }