Validate being called on structs that are nil

[blakerouse]

In the case that the reflect.Value is nil even if it implements the Validator interface Validate should not be called. Before 0.8.0 Validate would not be called, causing issues with existing beats and libbeat.

One example of an error in libbeat:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x70 pc=0x43c8945]

goroutine 44 [running]:
testing.tRunner.func1(0xc00016e500)
        /usr/local/go/src/testing/testing.go:874 +0x3a3
panic(0x451d380, 0x4a41c80)
        /usr/local/go/src/runtime/panic.go:679 +0x1b2
github.com/elastic/beats/libbeat/common/transport/tlscommon.(*CertificateConfig).Validate(...)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/libbeat/common/transport/tlscommon/types.go:256
github.com/elastic/beats/libbeat/common/transport/tlscommon.(*Config).Validate(0x0, 0x4522fa0, 0x0)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/libbeat/common/transport/tlscommon/config.go:97 +0x5
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.tryValidate(0x4522fa0, 0xc00046a290, 0x196, 0x4522fa0, 0xc00046a290)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/validator.go:123 +0xf3
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.tryRecursiveValidate(0x4522fa0, 0xc00046a290, 0x196, 0xc00016c400, 0x0, 0x0, 0x0, 0x0, 0x0)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/validator.go:163 +0x132
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.reifyGetField(0xc00015a900, 0xc00016c400, 0x0, 0x0, 0x0, 0x0, 0x44ade00, 0x3, 0x4522fa0, 0xc00046a290, ...)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/reify.go:317 +0x19a
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.reifyStruct(0xc00016c400, 0x4594f00, 0xc00046a1b0, 0x199, 0xc00015a900, 0x0, 0x0)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/reify.go:282 +0x939
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.reifyInto(0xc00016c400, 0x44df7a0, 0xc00046a1b0, 0x16, 0xc00015a900, 0x0, 0x4594f00)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/reify.go:172 +0x3fb
github.com/elastic/beats/vendor/github.com/elastic/go-ucfg.(*Config).Unpack(0xc00015a900, 0x44df7a0, 0xc00046a1b0, 0x4a4ea60, 0x3, 0x3, 0x24, 0xc000064e90)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/vendor/github.com/elastic/go-ucfg/reify.go:155 +0x103
github.com/elastic/beats/libbeat/common.(*Config).Unpack(...)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/libbeat/common/config.go:168
github.com/elastic/beats/libbeat/outputs/logstash.readConfig(0xc00015a900, 0x45be607, 0x8, 0x45bd0f3, 0x3, 0x0, 0x0, 0x45bd108, 0x3, 0x0, ...)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/libbeat/outputs/logstash/config.go:78 +0x171
github.com/elastic/beats/libbeat/outputs/logstash.TestConfig.func1(0xc00016e500)
        /Users/blake/Code/elastic/beats/src/github.com/elastic/beats/libbeat/outputs/logstash/config_test.go:90 +0x89
testing.tRunner(0xc00016e500, 0xc00046a120)
        /usr/local/go/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
        /usr/local/go/src/testing/testing.go:960 +0x350