[sr-dev] [kamailio/kamailio] db_redis: fix compilation warning (#1495)

CC (clang) [M db_redis.so] redis_table.o redis_table.c:728:27: warning: comparison of constant -1 with expression of type 'unsigned char' is always false [-Wtautological-constant-out-of-range-compare] if (c == EOF) { ~ ^ ~~~ redis_table.c:754:27: warning: comparison of constant -1 with expression of type 'unsigned char' is always false [-Wtautological-constant-out-of-range-compare] if (c == EOF) { ~ ^ ~~~ redis_table.c:794:40: warning: comparison of constant -1 with expression of type 'unsigned char' is always true [-Wtautological-constant-out-of-range-compare] if (c != '\n' && c != EOF) { ~ ^ ~~~ redis_table.c:807:20: warning: comparison of constant -1 with expression of type 'unsigned char' is always true [-Wtautological-constant-out-of-range-compare] } while (c != EOF); ~ ^ ~~~