flxdec: fix some warnings comparing unsigned < 0

flxdec: fix some warnings comparing unsigned < 0

bf43f44fcfada5ec4a3ce60cb374340486fe9fac was comparing an unsigned
expression to be < 0 which was always false.

gstflxdec.c: In function ‘flx_decode_brun’:
gstflxdec.c:322:33: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
         if ((glong) row - count < 0) {
                                 ^
gstflxdec.c:332:33: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
         if ((glong) row - count < 0) {
                                 ^

https://bugzilla.gnome.org/show_bug.cgi?id=774834