net/http: reject negative suffix-length Range:bytes=--N with 416 status code
Fixes the file server to reject requests of the form:
"Range": "bytes=--N"
where "-N" is a negative suffix-length as designated by the
grammar in RFC 7233 Section 2.1, "Byte-Ranges", which specifies
that suffix-length MUST be of the form 1*DIGIT aka a non-negative digit.
Thus requests such as:
"Range": "bytes=--2"
will be rejected with a "416 Range Not Satisfiable" response.