Fix warning (turning error) when compiling with -O3

In file included from /home/jmayer/work/wireshark/git/epan/wslua/lrexlib_glib.c:199:0:
/home/jmayer/work/wireshark/git/epan/wslua/lrexlib_algo.h: In function ‘generic_find_func’:
/home/jmayer/work/wireshark/git/epan/wslua/lrexlib_algo.h:486:14: error: ‘ud’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   TUserdata *ud;
              ^
/home/jmayer/work/wireshark/git/epan/wslua/lrexlib_algo.h: In function ‘algf_gsub’:
/home/jmayer/work/wireshark/git/epan/wslua/lrexlib_algo.h:281:14: error: ‘ud’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   TUserdata *ud;
              ^

Change-Id: I835103ea562ced44bc3cce5fadf6115476a78d0e
Reviewed-on: https://code.wireshark.org/review/3568
Reviewed-by: Jörg Mayer <jmayer@loplof.de>