Fix build when use Clang

libevent-client.c:355:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]
Fix build when use Clang

libevent-server.c:552:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]
Merge branch 'clang' of https://github.com/alagoutte/nghttp2 into alagoutte-clang
examples: Zero clear callbacks