t_zset.c/zslIsInLexRange: empty ranges condition correct

Mind for the case:
    
    int main(int argc, char *argv[])
    {
        char *test1 = "ababb";
        printf("%d\n", memcmp(test1+1, test1, 1));
        printf("%d\n", memcmp(test1+2, test1, 3));
        return 0;
    }
As `compareStringObjectsForLexRange()` calls `memcmp()` to compare two string.
The special case `memcpy("abb","aba",3) == 1` 
So the edge case may cause potential bugs. :-)