Add stopping in the debugger when nhassert() is hit in the windows port.

When stopping in the debugger after having called impossible, the windowing
state will have been modified since the assertion was hit.  This made
examining state that caused the nhassert to fire no longer possible.
To avoid this issue, we now detect the debugger and stop in the debugger
prior to impossible.

sys/winnt/winnt.c

@@ -726,9 +726,19 @@ nt_assert_failed(expression, filepath, line)
 {
     const char * filename;
 
-    /* get file name from path */
     filename = strrchr(filepath, '\\');
     filename = (filename == NULL ? filepath : filename + 1);
+
+    if (IsDebuggerPresent()) {
+        char message[BUFSIZ];
+        snprintf(message, sizeof(message), 
+            "nhassert(%s) failed in file '%s' at line %d",
+            expression, filename, line);
+        OutputDebugStringA(message);
+        DebugBreak();
+    }
+
+    /* get file name from path */
     impossible("nhassert(%s) failed in file '%s' at line %d",
                 expression, filename, line);
 }