Fixes a memory leak in SCAN, with OPT_SCAN_RETRY enabled.

When you instruct phpredis to retry the scan command in the event
of a non zero iterator but zero elements being returned, it was
leaking memory, as it did not free the previous result.

Addresses #501
Merge pull request #1 from nicolasff/master

Fixes a memory leak in SCAN, with OPT_SCAN_RETRY enabled.