In gridfs_store_stream, there are unchecked malloc and realloc calls.
if any of those fail with NULL, the following code will dereference NULL,
probably provoking a segfault:

if ( to_write < gfs->chunk_len ) {
if( gfs->pending_data )

{ gfs->pending_data = (char *)realloc((void *)gfs->pending_data, gfs->pending_len + to_write); memcpy( gfs->pending_data + gfs->pending_len, data, length ); }

else if (to_write > 0)

{ gfs->pending_data = (char *)malloc(to_write); memcpy( gfs->pending_data, data, length ); }

gfs->pending_len += length;
} else {

I didn't try to fix it because I suspect the function is incomplete.
Its return type is "bson", yet there is no return statement.