Before returning because of an memory allocation error, free the memory

already allocated to buf.

Found using clang's static analyzer - scan-build

Submitted by:	Thomas Rix <trix@juniper.net>
Reviewed by:	stevek
Approved by:	sjg (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D9852

usr.sbin/ppp/iface.c

@@ -145,6 +145,7 @@ iface_Create(const char *name)
       iface = (struct iface *)malloc(sizeof *iface);
       if (iface == NULL) {
         fprintf(stderr, "iface_Create: malloc: %s\n", strerror(errno));
+	free(buf);
         return NULL;
       }
       iface->name = strdup(name);