remove uncommon branch from asciiprot hot path

The \0 test in the loop was accounting for 2% of memcached's CPU usage
according to callgrind. strlen is an SSE4 instruction and can sniff out that
null byte quickly.

memcached.c

@@ -2390,28 +2390,34 @@ typedef struct token_s {
 static size_t tokenize_command(char *command, token_t *tokens, const size_t max_tokens) {
     char *s, *e;
     size_t ntokens = 0;
+    size_t len = strlen(command);
+    unsigned int i = 0;
 
     assert(command != NULL && tokens != NULL && max_tokens > 1);
 
-    for (s = e = command; ntokens < max_tokens - 1; ++e) {
+    s = e = command;
+    for (i = 0; i < len; i++) {
         if (*e == ' ') {
             if (s != e) {
                 tokens[ntokens].value = s;
                 tokens[ntokens].length = e - s;
                 ntokens++;
                 *e = '\0';
+                if (ntokens == max_tokens - 1) {
+                    e++;
+                    s = e; /* so we don't add an extra token */
+                    break;
+                }
             }
             s = e + 1;
         }
-        else if (*e == '\0') {
-            if (s != e) {
-                tokens[ntokens].value = s;
-                tokens[ntokens].length = e - s;
-                ntokens++;
-            }
+        e++;
+    }
 
-            break; /* string end */
-        }
+    if (s != e) {
+        tokens[ntokens].value = s;
+        tokens[ntokens].length = e - s;
+        ntokens++;
     }
 
     /*