MINOR: hlua: Don't needlessly copy lua strings in trash during args validation

MINOR: hlua: Don't needlessly copy lua strings in trash during args validation

Lua strings are NULL terminated. So in the hlua_lua2arg_check() function, used
to check arguments against the sample fetches specification, there is no reason
to copy these strings in a trash to add a terminating null byte.

In addition, when the array of arguments is built from lua values, we must take
care to count this terminating null bytes in the size of the buffer where a
string is stored. The same must be done when a sample is built from a lua value.

This patch may be backported to easy backports.

(cherry picked from commit fdea1b6319bc014320a18ba7b7043ba0694dd0dd)
[wt: backported for next commit]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit f657d400b442b83d1565579c2b78eba9d4e12abc)
Signed-off-by: Willy Tarreau <w@1wt.eu>