Added nhassert() support to tile2bmp.

win/share/tile2bmp.c

@@ -360,3 +360,10 @@ pixel (*pixels)[TILE_X];
         }
     }
 }
+
+/* nhassert_failed is called when an nhassert's condition is false */
+void nhassert_failed(const char * exp, const char * file, int line)
+{
+    Fprintf(stderr, "NHASSERT(%s) in '%s' at line %d\n", exp, file, line);
+    exit(EXIT_FAILURE);
+}