Parse errors on header tags are masked when using NewParserFromFile

[sirnewton01]

When parsing a non-conforming DICOM file that doesn't have the MetaElementGroupLength tag using NewParserFromFile (e.g. dicomutil) the error is masked by an interface conversion error.

panic: interface conversion: dicom.Parser is nil, not *dicom.parser

goroutine 1 [running]:
github.com/gradienthealth/dicom.NewParserFromFile(0x7ffeefbff937, 0x1e, 0x0, 0x2, 0x0, 0x0, 0x40)
	/Users/cmcgee/go/src/github.com/gradienthealth/dicom/parse.go:81 +0x19c
main.main()
	/Users/cmcgee/go/src/github.com/gradienthealth/dicom/dicomutil/dicomutil.go:74 +0x4dd

The cause is in the NewParserFromFile function that is trying to convert the interface, which can be nil, before checking the error from the call to NewParser() in parse.go line 81. Instead, the error should be checked before attempting to convert the interface. Alternatively, the second return value of the interface conversion could be checked to make sure that the conversion succeeds, avoiding the panic.