fix(fs): reject zero-length suffix byte ranges - #2460

ParseByteRange accepts the suffix range bytes=-0 and returns startPos = contentLength, endPos = contentLength - 1. The FS handler then replies 206 Partial Content with an empty body and an invalid Content-Range:

fs := &fasthttp.FS{Root: ".", AcceptByteRange: true}
// GET /LICENSE with "Range: bytes=-0"
// => 206, Content-Range: bytes 1158-1157/1158, Content-Length: 0

RFC 9110 §14.1.1 only treats a suffix range as satisfiable when its suffix-length is non-zero, so this should be 416 Range Not Satisfiable. Every other unsatisfiable range already gets that response through the error path. ParseByteRange now returns an error for a zero suffix length.

Added bytes=-0 to TestParseByteRangeError; it fails without the change. go test . and go vet . pass.