[core] remove assert in fdevent_unregister()
[core] remove assert in fdevent_unregister()

assumptions changed: fdn->events can be 0 with server streaming
request and/or response.

gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4