snmp.c: compilation fixes for AIX

snmp.c: compilation fixes for AIX

The C and C++ standards allows the character type char to be signed or
unsigned, depending on the platform and compiler. Most systems,
including x86 GNU/Linux and Microsoft Windows, use signed char, but
those based on PowerPC and ARM processors typically use unsigned char.

This patch fixes a "comparison is always true" warning on AIX (powerpc)
which leads the compilation to abort because of the -Werror flag.

Being unsigned by default, a char is always >0.

Signed-off-by: Aurelien Reynaud <collectd@wattapower.net>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>