Fix unsigned underflow in SimpleString::subString() on empty strings - #1889

SimpleString::subString(size_t beginPos, size_t amount) rejects an out-of-range beginPos with:

if (beginPos > size()-1) return "";

size() returns size_t (unsigned). When called on an empty string, size() is 0, so size()-1 underflows to SIZE_MAX. The check beginPos > SIZE_MAX is then false for essentially any beginPos, so the bounds check is silently defeated and the function falls through to:

SimpleString newString = getBuffer() + beginPos;

which reads out of bounds of the (1-byte, null-terminator-only) buffer of an empty SimpleString.

Repro (confirmed with a guard-page allocator that places an empty SimpleString's buffer directly against a PAGE_NOACCESS page):

SimpleString empty("");
SimpleString result = empty.subString(1, 1); // reads 1 byte past the buffer

This reliably crashes before the fix and returns "" after it.

Fix: change the check to beginPos >= size(), which is mathematically identical to the original for every size() >= 1 (so every existing non-empty-string call site is byte-for-byte unaffected) but no longer underflows when size() == 0.

Verified:

• Compiled a minimal repro harness against the unmodified sources: empty.subString(1, 1) segfaults before the fix.
• Recompiled the same harness against the fixed source: no crash, subString(1, 1) on an empty string correctly returns "".
• Rebuilt and ran the existing SimpleString test group (AllTests -g SimpleString, 130 tests / 238 checks) against both the original and fixed source — identical OK results before and after, so this is a pure bug fix with no behavior change for any existing test or non-empty-string caller.