Core (LV::Video): Fix PNG loader not resetting stream position on error right after reading signature.

Author: kaixiong

libvisual/libvisual/private/lv_video_png.cpp

@@ -62,6 +62,11 @@ namespace LV {
 
       png_byte signature[8];
       input.read (reinterpret_cast<char*> (signature), sizeof (signature));
+      input.seekg (saved_stream_pos);
+
+      if (!input) {
+          return nullptr;
+      }
 
       bool is_png = !png_sig_cmp (signature, 0, sizeof (signature));
 
@@ -100,6 +105,8 @@ namespace LV {
           return nullptr;
       }
 
+      input.seekg (sizeof (signature), std::ios::cur);
+
       png_set_read_fn (png_ptr, &input, handle_png_read);
 
       png_set_sig_bytes (png_ptr, sizeof (signature));