scte/35: fix sign warnings

This resolves the following (6) warnings, emitted when compiling the make target 'examples' without -Wno-sign-compare and with -Wall.

In file included from dvb_gen_si.c:44: ./bitstream/scte/35.h: In function ‘scte35_insert_validate’: ./bitstream/scte/35.h:550:22: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]

./bitstream/scte/35.h:578:25: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Wsign-compare]

./bitstream/scte/35.h:556:29: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]

./bitstream/scte/35.h:580:21: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Wsign-compare]

./bitstream/scte/35.h:561:25: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]

./bitstream/scte/35.h:578:25: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Wsign-compare]

This change ought not to change any logic, but merely preserve the existing one whilst formalizing the behavior in such a way that the compiler is made painfully aware of our intent prompting it to stop emitting the warnings.