m25p80: don't let rogue SPI controllers cause buffer overruns

In normal operation we should never attempt to put more
data into the data[] array than it can hold. However if the
SPI controller connected to us misbehaves then it can send
us a sequence of commands that attempt this. Since the
controller might be in the guest (if the hardware does SPI
via bit-banging), catch the possible overrun conditions and
reset the flash internal state, logging them as guest errors.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 20170107111631.24444-1-jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: rewrote commit message to be more exact about when
 this can happen]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
