Fixed a bug when writing the response

[lodi-g]

Changed the way to write response

- Writing len - 1, to avoid writing a \0 to stdout
- Using asprintf to build the url instead of a VLA

[amckinlay]

Why is the related change reducing the socket timeout to 500ms better? Why switch to asprintf()? It is not SUS.

+1 on this PR. nc termbin.com 9999 terminates with a null character. When the output of nc is piped directly to the clipboard, the termbin URL looks like:

When termbin is run from Vim it also shows the null character:

  1 hi
~

:w !{nc termbin.com 9999}
https://termbin.com/jtrg
^@
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