Potential serial input buffer overflow bug in EmonTxV4_config.ino

[nmcgann]

The serial input buffer in getSettings() can overflow with a long input sequence as the test if (idx<64) { is incorrect.

The buffer input is 64 characters long, but it should reserve 1 byte for the \0 string terminator. With 0-based addressing that means that only 0-62 are valid character addresses, so the test should be idx<63 to ensure the zero terminator is never overwritten with a character.