In _gettemp(), check that the length of the path doesn't exceed

MAXPATHLEN. Otherwise the path name (or part of it) may not fit to
carrybuf causing a buffer overflow.

PR:		bin/140228
Suggested by:	jilles

lib/libc/stdio/mktemp.c

@@ -116,6 +116,10 @@ _gettemp(path, doopen, domkdir, slen)
 
 	for (trv = path; *trv != '\0'; ++trv)
 		;
+	if (trv - path >= MAXPATHLEN) {
+		errno = ENAMETOOLONG;
+		return (0);
+	}
 	trv -= slen;
 	suffp = trv;
 	--trv;