Move the check whether the clear flag is set. This has 2 advantages

- when operating on a core file (-M) and -c is specified we don't clear
  the message buffer of the running system.
- if we don't have permission to clear the buffer print the error message
  only. That's what Linux does in this case, where this feature was ported
  from, and it ensures that the error message doesn't get lost in the noise.

Index: dmesg.c
===================================================================
--- dmesg.c	(revision 251616)
+++ dmesg.c	(working copy)
@@ -120,6 +120,9 @@
 			errx(1, "malloc failed");
 		if (sysctlbyname("kern.msgbuf", bp, &buflen, NULL, 0) == -1)
 			err(1, "sysctl kern.msgbuf");
+		if (clear)
+			if (sysctlbyname("kern.msgbuf_clear", NULL, NULL, &clear, sizeof(int)))
+				err(1, "sysctl kern.msgbuf_clear");
 	} else {
 		/* Read in kernel message buffer and do sanity checks. */
 		kd = kvm_open(nlistf, memf, NULL, O_RDONLY, "dmesg");
@@ -196,10 +199,6 @@
 		(void)strvisx(visbp, p, nextp - p, 0);
 		(void)printf("%s", visbp);
 	}
-	if (clear)
-		if (sysctlbyname("kern.msgbuf_clear", NULL, NULL, &clear, sizeof(int)))
-			err(1, "sysctl kern.msgbuf_clear");
-
 	exit(0);
 }