Unsanitised pattern input to string.gsub

[Danie-1]

When running tex4ebook with the option -B "epub-build" I got an error saying "attempt to index a nil value".

The root cause is in tex4ebook-exec_epub.lua, in the function remove_builddir, where a call is made to string.gsub with unsanitised user input:

return filename:gsub("^" .. builddir .. "/", "")

In this particular case, the cause is that "-" is a special character for lua patterns, so the pattern "^epub-build/" does not match for strings which start with "epub-build/". I will try to submit a pull request with a suggested fix soon.