fix(nrf54l15): don't write past String buffer when a grow fails (#11454)

* 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).

src/platform/nrf54l15/Arduino.h

@@ -599,8 +599,12 @@ class String
 
     void assign(const char *s, unsigned int n)
     {
-        if (n >= _cap)
-            reserve(n + 1);
+        // reserve() keeps the old (smaller) buffer on OOM, so a failed grow must abort the
+        // write: memcpy'ing n >= _cap bytes would overflow into adjacent heap.
+        if (n + 1 == 0)
+            return; // n + 1 would wrap
+        if (n >= _cap && !reserve(n + 1))
+            return;
         if (_buf) {
             memcpy(_buf, s, n);
             _buf[n] = 0;
@@ -612,21 +616,27 @@ class String
         if (!s || n == 0)
             return;
         unsigned newlen = _len + n;
-        if (newlen >= _cap)
-            reserve(newlen + 1);
+        if (newlen < _len || newlen + 1 == 0)
+            return; // length arithmetic wrapped
+        if (newlen >= _cap && !reserve(newlen + 1))
+            return; // OOM: keep the existing content intact instead of writing past the buffer
         if (_buf) {
             memcpy(_buf + _len, s, n);
             _len = newlen;
             _buf[_len] = 0;
         }
     }
-    void reserve(unsigned int n)
+    bool reserve(unsigned int n)
     {
+        if (n == 0)
+            return false;
         char *b = (char *)realloc(_buf, n);
         if (b) {
             _buf = b;
             _cap = n;
+            return true;
         }
+        return false;
     }
 };