Issue handling zero-length SYSEX message

[mchesser]

The code doesn't handle zero length SYSEX messages correctly. Sending the message [START_SYSEX, END_SYSEX]/[0xf0, 0xf7], causes processSysexMessage to be called (here) with sysexBytesRead == 0.

I think this causes two issues:

• First, this will causes the previous SYSEX command to be executed, because storedInputData[0] is not valid:

https://github.com/firmata/ConfigurableFirmata/blob/c759e5a0375bdbb89efb0c7efe640dccd3305f65/src/ConfigurableFirmata.cpp#L216

• Second, an underflow occurs when computing sysexBytesRead - 1. If the previous SYSEX command was as STRING_DATA command then the code here ConfigurableFirmata.cpp:229 could overflow the storedInputData buffer (though only without LARGE_MEM_DEVICE defined, since the index is a byte).

In practice, I guess clients shouldn't generate these messages but I thought it was worth reporting.

[pgrawehr]

Thanks for the report.

I agree this is a minor issue, but should still be looked at.