Fix count overflow on readBytes/readWords

[nekomona]

No description provided.

[nekomona]

Detailed description in #751.
The patch fixed the issue in Arduino lib. For other MCUs it may need further verification.

[jrowberg]

Good catch on this...I think most people must not have tried to read that many bytes all at once. The solution here is an improvement over what was previously implemented, but it still leaves ambiguity about what the return code means when the raw value is above +127. If you read 168 bytes successfully, but compare the function return value with a simple result < 0 kind of check, you'll incorrectly detect an error because the function's actual return type is still int8_t. Maybe the cleaner fix is to leave the max read length as a uint8_t but change the count and return type to int16_t.

[nekomona]

Changing return type should be a clean fix for the library but I'm not sure if it would hit existing programs with return values stored in a int8_t variable. Guess I'll leave the issue open to mark that.