unpack-trees: avoid array out-of-bounds error

The loop in warn_conflicted_path() that checks for the count of
entries with the same path uses "i+count" for the array
entry. However, the loop only verifies that the value of count is
below the array size. Fix this by adding i to the condition.

I hit this condition during a test of the in-tree sparse-checkout
feature, so it is exercised by the end of the series.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
[jc: readability fix]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -562,11 +562,11 @@ static int warn_conflicted_path(struct index_state *istate,
 
 	add_rejected_path(o, WARNING_SPARSE_UNMERGED_FILE, conflicting_path);
 
-	/* Find out how many higher stage entries at same path */
-	while (++count < istate->cache_nr &&
-	       !strcmp(conflicting_path,
-		       istate->cache[i+count]->name))
-		/* do nothing */;
+	/* Find out how many higher stage entries are at same path */
+	while ((++count) + i < istate->cache_nr &&
+	       !strcmp(conflicting_path, istate->cache[count + i]->name))
+		; /* do nothing */
+
 	return count;
 }