Dump ziplist hex value on failed assertion.

The ziplist -> hashtable conversion code is triggered every time an hash
value must be promoted to a full hash table because the number or size of
elements reached the threshold.

If a problem in the ziplist causes the same field to be present
multiple times, the assertion of successful addition of the element
inside the hash table will fail, crashing server with a failed
assertion, but providing little information about the problem.

This code adds a new logging function to perform the hex dump of binary
data, and makes sure that the ziplist -> hashtable conversion code uses
this new logging facility to dump the content of the ziplist when the
assertion fails.

This change was originally made in order to investigate issue #547.

src/redis.h

@@ -1128,4 +1128,5 @@ sds genRedisInfoString(char *section);
 void enableWatchdog(int period);
 void disableWatchdog(void);
 void watchdogScheduleSignal(int period);
+void redisLogHexDump(int level, char *descr, void *value, size_t len);
 #endif