Add checks for ReadStream offset boundary conditions

Several checks for ReadStream with offset around boundary conditions were
missing. The new checks ensure negative offsets are detected and io.EOF is
returned properly when trying to read past the end of a file. The filesystem
and inmemory driver have been updated accordingly.

An outline of missing checks for List are also part of this commit. Action will
be taken here based on discussion in issue #819.

storagedriver/inmemory/driver.go

@@ -83,6 +83,10 @@ func (d *Driver) ReadStream(path string, offset int64) (io.ReadCloser, error) {
 	d.mutex.RLock()
 	defer d.mutex.RUnlock()
 
+	if offset < 0 {
+		return nil, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
+	}
+
 	path = d.normalize(path)
 	found := d.root.find(path)