cast signed/unsigned before comparison
cast signed/unsigned before comparison

fixes this compilation error:

    sg_zing.c: In function ‘sg_zing__check_field__string’:
    sg_zing.c:2194: error: comparison between signed and unsigned integer expressions
    sg_zing.c:2205: error: comparison between signed and unsigned integer expressions

Signed-off-by: Ævar Arnfjörð Bjarmason <0135fcc047f75af264c800977a73e353c6ba842d@gmail.com>

avar/veracity,avar/veracity,avar/veracity