sdstrim remove excessive check (#4045)
sdstrim remove excessive check (#4045)

there is no need to compare the value of ep and sp
```
    sp = start = s;
    // the only way that make ep > sp is sdslen(s) == 0
    // so when ep > sp,must exist ep-sp == -1
    ep = end = s+sdslen(s)-1;
    while(sp <= end && strchr(cset, *sp)) sp++;
    while(ep > sp && strchr(cset, *ep)) ep--;
    // -1 + 1 already equals 0
    len = (sp > ep) ? 0 : ((ep-sp)+1);
```

Signed-off-by: Bo Cai <9118b02aa2c23efb995d5afd89066e201eb6daac@gmail.com>
oranagra/redis,charsyam/redis,antirez/redis,ofirluzon/redis,charsyam/redis,yossigo/redis,neomantra/redis,neomantra/redis,soloestoy/redis,oranagra/redis,soloestoy/redis,ofirluzon/redis,yossigo/redis,soloestoy/redis,yossigo/redis,soloestoy/redis,PKRoma/redis,oranagra/redis,soloestoy/redis,charsyam/redis,antirez/redis,ofirluzon/redis,neomantra/redis,PKRoma/redis,charsyam/redis,oranagra/redis,antirez/redis,yossigo/redis,ofirluzon/redis,neomantra/redis,oranagra/redis,neomantra/redis,PKRoma/redis,yossigo/redis,PKRoma/redis,charsyam/redis,PKRoma/redis,antirez/redis,ofirluzon/redis