GetCellValue is concurrency unsafe, maybe occur `fatal error: concurrent map writes`

[jarcoug]

https://github.com/qax-os/excelize/blob/d133dc12d774dcf944b36720f25e1dd945dda63f/excelize.go#L280C3-L280C3

f.xmlAttr is define Map, so concurrent invoke GetCellValue, maybe occur fatal error: concurrent map writes

[xuri]

Thanks for your issue. This issue has been fixed, please try to upgrade the master branch code, and this patch will be released in the next version.