Fix for gridfs file seek

gridfs file seek had incorrect handling for detecting page migration.
Fixed a potential segfault and the underlying conditional.

Fixes #155

Author: hanumantmk

src/mongoc/mongoc-gridfs-file.c

@@ -720,14 +720,15 @@ mongoc_gridfs_file_seek (mongoc_gridfs_file_t *file,
 
    BSON_ASSERT (file->length > (int64_t)offset);
 
-   if (offset % file->chunk_size != file->pos % file->chunk_size) {
+   if (offset / file->chunk_size != file->pos / file->chunk_size) {
       /** no longer on the same page */
 
       if (file->page) {
          if (_mongoc_gridfs_file_page_is_dirty (file->page)) {
             _mongoc_gridfs_file_flush_page (file);
          } else {
             _mongoc_gridfs_file_page_destroy (file->page);
+            file->page = NULL;
          }
       }
 

tests/test-mongoc-gridfs.c

@@ -250,6 +250,13 @@ test_read (void)
    assert (memcmp (iov[0].iov_base, "Bacon ipsu", 10) == 0);
    assert (memcmp (iov[1].iov_base, "m dolor si", 10) == 0);
 
+   assert (mongoc_gridfs_file_seek (file, 0, SEEK_SET) == 0);
+   r = mongoc_gridfs_file_readv (file, iov, 2, 20, 0);
+
+   assert (r == 20);
+   assert (memcmp (iov[0].iov_base, "Bacon ipsu", 10) == 0);
+   assert (memcmp (iov[1].iov_base, "m dolor si", 10) == 0);
+
    mongoc_gridfs_file_destroy (file);
 
    drop_collections (gridfs, &error);