.netrc parser out-of-bounds access

curl can be told to parse a .netrc file for credentials. If that file ends
in a line with consecutive non-white space letters and no newline, curl could
read past the end of the stack-based buffer, and if the read works, write a
zero byte possibly beyond its boundary.

This does in most cases cause a segfault or similar, but circumstances might
also cause different outcomes.

If a malicious user can provide a custom netrc file to an application or
otherwise affect its contents, this flaw could be used as denial-of-service.