Fixes a panic that can be caused by bad input data

[ssoroka]

Found this panic using go-fuzz.

Relevant panic output was:

panic: runtime error: index out of range

goroutine 1 [running]:
github.com/rwcarlsen/goexif/exif.(*parser).Parse(0xb0c2b0, 0xc4200161b0, 0xc4200161e0, 0x0)
github.com/rwcarlsen/goexif/exif/exif.go:147 +0x4c5
github.com/rwcarlsen/goexif/exif.Decode(0xac2ba0, 0xc420016090, 0xc42054fab8, 0x1051e, 0xc420016090)
github.com/rwcarlsen/goexif/exif/exif.go:287 +0x6ea
[...]

I can provide the input that triggered the crash if required.

[ssoroka]

resolves #39

[xor-gate]

@ssoroka It seems the author is not very active in this project anymore, many forks have been created. I would like to make this stable and I created goexif2 which integrates your patch: xor-gate@c654e02