awesome.spawn(): Check table arguments better

awesome.spawn(): Check table arguments better

Previously, any not-string-convertible entry in the array argument would result
in lua_tostring() returning NULL which g_strdup() would pass through. Thus, we
would end up with a NULL entry in an array whose end is marked with a NULL
entry. This mainly means that we had a memory leak.

Fix this by actually verifying that we only have strings in the table that we
are looking at.

Signed-off-by: Uli Schlachter <psychon@znc.in>