store: don't underflow pointer

when number of snips is 0, `end - 1` will go one below the
buffer. computing out of bound pointer (with the exception of
one past the end) is technically UB [0] so avoid it by returning
early when nr_snips == 0.

0: https://port70.net/~nsz/c/c11/n1570.html#6.5.6p9

src/store.c

@@ -562,7 +562,7 @@ int cs_add(struct clip_store *cs, const char *content, uint64_t *out_hash) {
  */
 bool cs_snip_iter(struct ref_guard *guard, enum cs_iter_direction direction,
                   struct cs_snip **snip) {
-    if (guard->status < 0) {
+    if (guard->status < 0 || guard->cs->header->nr_snips == 0) {
         return false;
     }
 
@@ -578,7 +578,7 @@ bool cs_snip_iter(struct ref_guard *guard, enum cs_iter_direction direction,
         *snip = direction == CS_ITER_NEWEST_FIRST ? end - 1 : start;
     }
 
-    return *snip >= start && *snip < end;
+    return *snip < end;
 }
 
 /**