Code cleanup, resolve an issue identified by cppcheck (#4373)

[src/bitops.c:512] -> [src/bitops.c:507]: (warning) Either the condition 'if(o&&o->encoding==1)' is redundant or there is possible null pointer dereference: o.

This function has checks for `o` to be null or non-null, so it is odd that it accesses it first..

src/bitops.c

@@ -508,7 +508,7 @@ robj *lookupStringForBitCommand(client *c, uint64_t maxbit, int *created) {
  * If the source object is NULL the function is guaranteed to return NULL
  * and set 'len' to 0. */
 unsigned char *getObjectReadOnlyString(robj *o, long *len, char *llbuf) {
-    serverAssert(o->type == OBJ_STRING);
+    serverAssert(!o || o->type == OBJ_STRING);
     unsigned char *p = NULL;
 
     /* Set the 'p' pointer to the string, that can be just a stack allocated