libc/minimal: snprintf(): KILL negative len parameter

snprintf() implements the ability to foce a negative value through the
(unsigned) size_t len parameter to allow the formatter to use a
maximum size string.

This is point less, we don't have as much memory and this is a recipe
for all kinds of vulnerabilities.

Kill the whole thing, the testcase it represents and thank Coverity
for finding this thing. Whatever use it had before, it has no more.

Change-Id: If422246548664699d8aa328a1b9304ef13cab7ea
Coverity-ID: 131625
Coverity-ID: 131626
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>