third_party/libpng: update to 1.2.54

third_party/libpng/pngpread.c

Index: third_party/libpng/pngpread.c
diff --git a/third_party/libpng/pngpread.c b/third_party/libpng/pngpread.c
index 5af209d4d533be5c16288161829c4379379c8f84..419995eab9fc7ab04aa9bbc90e9122b3f514bcd0 100644
--- a/third_party/libpng/pngpread.c
+++ b/third_party/libpng/pngpread.c
@@ -685,9 +685,12 @@ png_push_save_buffer(png_structp png_ptr)
         png_free(png_ptr, old_buffer);
         png_error(png_ptr, "Insufficient memory for save_buffer");
       }
-      png_memcpy(png_ptr->save_buffer, old_buffer, png_ptr->save_buffer_size);
-      png_free(png_ptr, old_buffer);
-      png_ptr->save_buffer_max = new_max;
+      else
+      {
+        png_memcpy(png_ptr->save_buffer, old_buffer, png_ptr->save_buffer_size);
+        png_free(png_ptr, old_buffer);
+        png_ptr->save_buffer_max = new_max;
+      }
    }
    if (png_ptr->current_buffer_size)
    {