[FAT32] fix corruption when appending to empty file #369
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When creating a new file, the start cluster in the directory entry is set to all zeroes. However, the context's state is set up so that the first byte written to the file causes the driver to allocate a new cluster.
If no byte is written to the file, this is never changed.
Prior to this change, when opening an existing file, the start cluster was trusted to point to the sector that the file's data began on. Unfortunately since this was the value $00000000, which is an alias for the root directory's start cluster, the next write would clobber the root directory and trash the filesystem entirely.
Rather than unconditionally trusting that the file has data and opening the first cluster, we now handle the case correctly when the opened file's size is 0.