luaconf.h warning when building

[josellausas]

When trying to build from source I repeatedly get the following warning:

torch-ios/exe/lua/src/luaconf.c:146:18: warning:
address of 'sb.buffer' will always evaluate to 'true'
[-Wpointer-bool-conversion]
while (&sb.buffer && sb.len>0 && sb.buffer[sb.len-1]=='/')