fix overlapped memcpy in heapextract

--- a/src/fingerprint.c 2013-07-04 22:53:25.674741538 +0200
+++ b/src/fingerprint.c	2013-07-04 22:54:09.009741166 +0200
@@ -254,7 +254,10 @@
     p = &(t->heap[0]);
 
     memcpy(item, p, sizeof(entry_t));
-    memcpy(&(t->heap[0]), &(t->heap[t->size - 1]), sizeof(entry_t));
+    if (t->size > 1)
+    {
+        memcpy(&(t->heap[0]), &(t->heap[t->size - 1]), sizeof(entry_t));
+    }
 
     siftdown(t, t->size, 0);
     t->size--;