Added nhassert to core.

src/pline.c

@@ -582,4 +582,10 @@ VA_DECL(const char *, str)
 #endif
 }
 
+/* nhassert_failed is called when an nhassert's condition is false */
+void nhassert_failed(const char * exp, const char * file, int line)
+{
+    impossible("NHASSERT(%s) in '%s' at line %d", exp, file, line);
+}
+
 /*pline.c*/