Prevent overread when scanning the message buffer

This change avoids the possibility of advancing past the null
terminator, by always testing the value at the pointer before
advancing the pointer.

While repairing this, I reconciled the code sections that read the
priority, sequence, and timestamp, so that they handle the pointer in
exactly the same way. This makes the source easier to maintain.

src/syslogd.c

@@ -1321,11 +1321,12 @@ void printsys(char *msg)
 
 		if (*p == '<') {
 			/* /proc/klog or *BSD /dev/klog */
+			p++;
 			buffer.pri = 0;
-			while (isdigit(*++p))
-				buffer.pri = 10 * buffer.pri + (*p - '0');
+			while (isdigit(*p))
+				buffer.pri = 10 * buffer.pri + (*p++ - '0');
 			if (*p == '>')
-				++p;
+				p++;
 		} else if (isdigit(*p)) {
 			/* Linux /dev/kmsg: "pri,seq#,msec,flag[,..];msg" */
 			time_t now;
@@ -1334,8 +1335,8 @@ void printsys(char *msg)
 			buffer.pri = 0;
 			while (isdigit(*p))
 				buffer.pri = 10 * buffer.pri + (*p++ - '0');
-
-			p++;	      /* skip ',' */
+			if (*p == ',')
+				p++;
 
 			/* seq# */
 			while (isdigit(*p))
@@ -1351,8 +1352,8 @@ void printsys(char *msg)
 					return;
 			}
 			sys_seqno = seqno;
-
-			p++;	      /* skip ',' */
+			if (*p == ',')
+				p++;
 
 			/* timestamp */
 			while (isdigit(*p))