solve issue #29 (from tcatm): don't bail out without nl80211

[matwei]

With this change the program just prints an error message when there is no support for nl80211 in the kernel.

To work properly it is checked wether ctx->wifistations_ctx.fd is initialized (!= 0) before polling at this file descriptor in function loop().

[matwei]

I addressed the comments from @NeoRaider in Version 3b146ba