Server should not panic when using a non-existent auth provider

[cheungpat]

When calling auth:login with a non-existent auth provider, skygear-server should not panic

• Skygear Server Date/Version: 0.9.0

Actual Results

time="2016-03-23T10:24:34Z" level=error msg="/go/src/github.com/oursky/skygear/router/errors.go:61 (0x5602e2)
    errorFromRecoveringPanic: log.Errorf(\"%s\", debug.Stack())
/go/src/github.com/oursky/skygear/router/router.go:101 (0x56621b)
    (*Router).ServeHTTP.func1: resp.Err = errorFromRecoveringPanic(r)
/usr/local/go/src/runtime/asm_amd64.s:437 (0x466ace)
    call32: CALLFN(·call32, 32)
/usr/local/go/src/runtime/panic.go:423 (0x437069)
    gopanic: reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
/usr/local/go/src/runtime/panic.go:42 (0x435729)
    panicmem: panic(memoryError)
/usr/local/go/src/runtime/sigpanic_unix.go:24 (0x44be2a)
    sigpanic: panicmem()
/go/src/github.com/oursky/skygear/handler/auth.go:143 (0x50631a)
    (*SignupHandler).Handle: principalID, authData, err := authProvider.Login(p.AuthData)
/go/src/github.com/oursky/skygear/router/router.go:142 (0x5651d2)
    (*Router).ServeHTTP: handler.Handle(payload, &resp)
/usr/local/go/src/net/http/server.go:1699 (0x49b0ad)
    (*ServeMux).ServeHTTP: h.ServeHTTP(w, r)
/go/src/github.com/oursky/skygear/router/logging.go:103 (0x563670)
    (*LoggingMiddleware).ServeHTTP: l.Next.ServeHTTP(rlogger, r)
/usr/local/go/src/net/http/server.go:1862 (0x49babe)
    serverHandler.ServeHTTP: handler.ServeHTTP(rw, req)
/usr/local/go/src/net/http/server.go:1361 (0x49930e)
    (*conn).serve: serverHandler{c.server}.ServeHTTP(w, w.req)
/usr/local/go/src/runtime/asm_amd64.s:1721 (0x468e21)
    goexit: BYTE    $0x90   // NOP
"
time="2016-03-23T10:24:34Z" level=error msg="panic occurred while handling request" recovered="runtime error: invalid memory address or nil pointer dereference"

[cheungpat]

fixed in 9756437