ppp: Fix seg fault when using pppol2tp

PPPD crashes (SEGV) when the dump or dryrun options are specified and an option
is internally defined as "o_special" with an option flag of "OPT_A2STRVAL".
As the option value is not saved when the parameter is processed, a reference
to the option will result into a crash (e.g. when printing).

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45263 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/network/services/ppp/patches/540-save-pppol2tp_fd_str.patch

+--- a/pppd/plugins/pppol2tp/pppol2tp.c
++++ b/pppd/plugins/pppol2tp/pppol2tp.c
+@@ -148,6 +148,10 @@ static int setdevname_pppol2tp(char **ar
+ 		fatal("PPPoL2TP kernel driver not installed");
+ 	}
+ 
++	pppol2tp_fd_str = strdup(*argv);
++	if (pppol2tp_fd_str == NULL)
++		novm("PPPoL2TP FD");
++
+ 	/* Setup option defaults. Compression options are disabled! */
+ 
+ 	modem = 0;