mmap fails when HAVE_MEM_MMAP_ANON because of zero length
Because c->file.copy.length is not changed from 0 the mmap fails. If if it worked the assert further ahead would fail.
This patch fixes that.
To see if afected, search log files for:
network_posix_aio.c.358: (error) the mmap() failed, no way for a fallback:
connections.c.1370: (trace) (network-subsys sent us a fatal-error)
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