sCalc: fix out-of-bounds read in lrc() on an empty operand

[physwkim]

Problem

lrc() in sCalcPerform.c loops with

for (i=0, lrc=0; i<strlen(rawInput)-1; i+=2) {
    lrc += hex(rawInput[i])*0x10 + hex(rawInput[i+1]);

strlen() returns size_t (unsigned). For an empty operand,
strlen(rawInput)-1 wraps to SIZE_MAX, so i < SIZE_MAX is always
true and the loop reads rawInput[] far out of bounds.

The LRC and AMODBUS operators call lrc() with whatever string is
on the stack (case LRC: / case AMODBUS: in sCalcPerform.c), so an
scalcout expression like LRC('') — or LRC applied to an empty
string field — crashes the IOC with SIGSEGV.

Fix

Compute the length once into a signed int and loop while i+1 < len.
An empty operand now yields an empty loop (LRC "00"); non-empty
operands are unchanged. The sibling xor8() already guards len == 0;
this brings lrc() in line.

Test

Adds two scalcTest cases:

• LRC('') → "00"
• LRC('F7031389000A') → "60" (the LRC 0x60 from the AMODBUS
  worked example in the source comment)

Without the fix scalcTest terminates with SIGSEGV on the empty-operand
case; with the fix the full suite passes (145/145).