Tony Nicholson e1ce9f4f70 sim_imd: Check for EOF when reading IMD track headers ...

The recent change to check that the IMD file track header record is 5
bytes introduced a problem with the end-of-file triggering an open file
error - instead of the previous behaviour where the number of bytes in
the track header was ignored and end-of-file was checked.

I noticed this after fetching and compiling the latest simh altairz80 - it
was refusing to my mount floppy disk IMD image files (which I had made
from 8-inch floppies back in 2006).

```
maxi:s100 tony$ altairz80 cpm3bk.ini

Altair 8800 (Z80) simulator V4.0-0 Current        simh git commit id: d3f1ee09
Console escape is CTRL-\
I8272: IMD disk corrupt.
/Users/tony/s100/cpm3bk.ini-58> att disk1a0 /Users/tony/s100/C3BKSIMH.IMD
File open error
```

Looking at the commit history, I saw the change by Howard Harte to
sim_imd.c to "Resolve CID 1502448, 1502460" - presumably to fix the
call to sim_fread() that was not checking the returned result.
Howard's "fix" broke the parsing of the IMD file sector headers by
not checking for an end-of-file condition (at the end of file it
would read 0 bytes and this was now treated as a fatal error - whereas
the old code had a check for end-of-file after the call to sim_fread()).

This commit now detects end-of-file correctly, as well as verifying
the track header is 5 bytes.

2022-05-01 08:01:18 +10:00

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