Fix syncd crashing due to invalid length being used for string printing

PR #801 added some code to convert binary strings to be printable
strings. However, one code path used the raw value of `len` without
sanity checking as the length of the string, when the `length()`
function should have been used instead.

Sample syncd backtrace:

```

(gdb) print *this
$22 = {temp = 0x55feeaced250 "*3\r\n$6\r\nSCRIPT\r\n$4\r\nLOAD\r\n$863\r\nlocal keys = redis.call('KEYS', KEYS[1])\nlocal n = table.getn(keys)\n\nfor i = 1, n do\n   if KEYS[2] == \"\" and KEYS[3] == \"1\" then\n      redis.call('DEL', keys[i])  \n   e"..., len = -355539936}
```

Fix this by using the `length()` function.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

common/rediscommand.cpp

@@ -130,7 +130,7 @@ int RedisCommand::appendTo(redisContext *ctx) const
 
 std::string RedisCommand::toPrintableString() const
 {
- return binary_to_printable(temp, len);
+ return binary_to_printable(temp, length());
 }
 
 const char *RedisCommand::c_str() const