bug#8238: possibly uninitialized variable in gif_load

[Paul Eggert <eggert@cs.ucla.edu>]

I found this problem by code inspection.

The following code in the Emacs trunk src/image.c's gif_load
function is suspicious, because it uses a variable transparent_p
that appears to be uninitialized in some cases:

  boolean transparent_p;
  ...
  for (i = 0; i < gif->SavedImages[ino].ExtensionBlockCount; i++)
    if ((gif->SavedImages[ino].ExtensionBlocks[i].Function
	 == GIF_LOCAL_DESCRIPTOR_EXTENSION)
	&& gif->SavedImages[ino].ExtensionBlocks[i].ByteCount == 4
	/* Transparency enabled?  */
	&& gif->SavedImages[ino].ExtensionBlocks[i].Bytes[0] & 1)
      {
	transparent_p = 1;
	...
      }
  ...
  if (gif_color_map)
    for (i = 0; i < gif_color_map->ColorCount; ++i)
      {
	if (transparent_p && transparency_color_index == i)

The code never sets transparent_p to any value other than 1,
but sometimes transparent_p appears to be uninitialized.

I'm filing a bug report so that someone who is more expert in this
code can take a look at it.  In the meantime, I plan to work around
the problem by initializing the variable to 0.

I'm CC'ing this to Julien Danjou, who committed the code in question.