hw/block/fdc-isa: Assert that isa_fdc_get_drive_max_chs() found something

Coverity complains about an overflow in isa_fdc_get_drive_max_chs()
that can happen if the loop over fd_formats never finds a match,
because we initialize *maxc to 0 and then at the end of the
function decrement it.

This can't ever actually happen because fd_formats has at least
one entry for each FloppyDriveType, so we must at least once
find a match and update *maxc, *maxh and *maxs. Assert that we
did find a match, which should keep Coverity happy and will also
detect possible bugs in the data in fd_formats.

Resolves: Coverity CID 1547663
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240731143617.3391947-6-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/block/fdc-isa.c

@@ -147,6 +147,8 @@ static void isa_fdc_get_drive_max_chs(FloppyDriveType type, uint8_t *maxc,
             *maxs = fdf->last_sect;
         }
     }
+    /* fd_formats must contain at least one entry per FloppyDriveType */
+    assert(*maxc);
     (*maxc)--;
 }