_sbrk has no bounds checking

_sbrk() in syscalls_sam3.c, does not contain any checks to see if the code is trying to acquired non-existent memory. This can lead malloc() and thus new to return pointers to either non-existent memory, or memory that belongs to the stack.

See https://forum.arduino.cc/t/ungraceful-handling-of-heap-depletion/1023509

Start in syscalls_sam3.c at _sbrk() and trace how malloc() and new obtain memory on the SAM3 platform. Reproduce heap exhaustion if possible, then verify that requests cannot return pointers to nonexistent memory or the stack.

Written by the indexing model from the issue text.