Fix potential memory leak when writing WebP throws (#2600)

dlib/image_saver/save_webp.cpp

@@ -74,8 +74,11 @@ namespace dlib {
             {
                 fout.write(reinterpret_cast<char*>(output), output_size);
                 if (!fout.good())
+                {
+                    WebPFree(output);
                     throw image_save_error("Error while writing WebP image to " + filename + ".");
                 }
+            }
             else
             {
                 throw image_save_error("Error while encoding WebP image to " + filename + ".");