* fix(nrf54l15): don't write past String buffer when a grow fails
reserve() correctly keeps the old buffer when realloc returns NULL, but
returned void, and assign()/concat() proceeded to memcpy with
n >= _cap anyway - a heap overflow of up to n+1-_cap bytes into
adjacent allocations. On this Zephyr target allocation failure is a
realistic condition, and the result was heap corruption instead of a
clean no-op.
reserve() now reports success and the callers leave the string
unchanged when the grow fails.
* fix(nrf54l15): guard String length arithmetic against wraparound
Per review: reject size requests whose n+1 / _len+n arithmetic would
wrap before they reach the capacity check, and make reserve(0) fail
without calling realloc (realloc(p, 0) would free the buffer and
return NULL, leaving _buf dangling).
Ben Meadors
a41ddec1a7