Fix NikonPictureControl1Directory parsing

[drewnoakes]

Fix NikonPictureControl1Directory string reading. These fixed-width, null-terminated strings. We don't want those nulls in the string we return.

Also some minor tidying and adding a unit test.

[gtiwari333]

@drewnoakes i am not sure how .NET works. but the code looks identical in Java and .NET.

Based on what i understand, this change will not read the value correctly. It will ONLY work if there are no null-terminated string in metadata.

        directory.Set(TagPictureControlVersion, reader.GetNullTerminatedStringValue(4));
        directory.Set(TagPictureControlName, reader.GetNullTerminatedStringValue(20));
        directory.Set(TagPictureControlBase, reader.GetNullTerminatedStringValue(20));

The issue is similar to what i described on drewnoakes/metadata-extractor#636 (comment).

In order to fix it, we need to either check the length of returned string and move the cursor after each read. Or, use a code similar to getNullTerminatedStringAndSkipToNextPosition.

[drewnoakes]

You're right, thanks. Fixed in #350 specifically e7cbf06 and bff31ad. I'll review the Java version, run the regression tests and get a fix up and another release out.