missing check in `epic_uart_write_str`

probably not a problem so far, but passing a negative length to epic_uart_write_str can lead to either a call to xStreamBufferSendFromISR or xStreamBufferSend with this negative length being (implicitely) cast to the unsigned size_t type. Maybe the length should be of an unsigned type to begin with? (or the check in the beginning of the function changed to check for <= 0 instead of ==0 ?)