Added error when opening multiple files with a statically allocated buffer
Added error when opening multiple files with a statically allocated buffer

Opening multiple files simultaneously is not supported without dynamic memory,
but the previous behaviour would just let the files overwrite each other, which
could lead to bad errors down the line

found by husigeza
geky/littlefs,littlefs-project/littlefs,geky/littlefs,littlefs-project/littlefs,geky/littlefs,geky/littlefs