waf: add -fcheck-new to g++ build

this ensures the compiler doesn't assume that new always returns a
non-NULL value. Without this the compiler may remove the error path in
code like this:

```
MyObject *x = new MyObject;
if (x == nullptr) {
  ::printf("Alloc failed\n");
}
```

the reason it can do this is the new operator is marked as throwing an
exception on failure, which means the error path is unreachable. As we
don't have C++ exceptions in ArduPilot could (and do!) have code that
ends up losing protection against allocation failures

Tools/ardupilotwaf/boards.py

@@ -249,6 +249,9 @@ class Board:
                 env.CXXFLAGS += [
                     '-Werror=implicit-fallthrough',
                 ]
+            env.CXXFLAGS += [
+                '-fcheck-new',
+            ]
 
         if cfg.env.DEBUG:
             env.CFLAGS += [