fsck_ffs: Don't overrun mount device buffer

Maybe this case is impossible.  Either way, when attempting to "/dev/"-prefix a
non-global device name, check that we do not overrun the f_mntfromname buffer.

In this case, truncating (with strlcpy or similar) would not be useful, since
the f_mntfromname result of getmntpt() is passed directly to open(2) later.

Reported by:	Coverity
CID:		1006789
Sponsored by:	EMC / Isilon Storage Division

sbin/fsck_ffs/main.c

@@ -644,6 +644,9 @@ getmntpt(const char *name)
 		statfsp = &mntbuf[i];
 		ddevname = statfsp->f_mntfromname;
 		if (*ddevname != '/') {
+			if (strlen(_PATH_DEV) + strlen(ddevname) + 1 >
+			    sizeof(statfsp->f_mntfromname))
+				continue;
 			strcpy(device, _PATH_DEV);
 			strcat(device, ddevname);
 			strcpy(statfsp->f_mntfromname, device);