use png_sig_cmp()

It seems that libpng 1.4 got rid of png_check_sig()
(http://www.libpng.org/pub/png/src/libpng-1.2.x-to-1.4.x-summary.txt)

The CHANGES doc that came with my libpng says that it was already
obsolete by at least January 1999 (v. 1.0.2a).

I checked 1.0.0 from March 98, and it has png_sig_cmp.
I checked ummm... 0.89c, I think it was (it was from May 96, in any case),
 and it did not have it yet.

I imagine we can ignore pre-1.0 from more than twelve years ago...

2 files changed, 3 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index 0e846755..13848071 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,7 +6,8 @@ dillo-2.2.1 [not released yet]
 +- Configurable User-Agent HTTP header.
    Patch: Alexander Voigt, corvid
 +- Include Accept header in HTTP queries.
-   Patch: corvid
+ - Don't use obsolete png_check_sig().
+   Patches: corvid
 
 -----------------------------------------------------------------------------
 
diff --git a/src/png.c b/src/png.c
index 353ebaa3..6a5f4422 100644
--- a/src/png.c
+++ b/src/png.c
@@ -345,8 +345,7 @@ static void Png_write(DilloPng *png, void *Buf, uint_t BufSize)
             return;            /* need MORE data */
          }
          /* check the image signature - DON'T update ipbufstart! */
-         if (!png_check_sig(png->ipbuf, DATASIZE)) {
-            /* you lied to me about it being a PNG image */
+         if (png_sig_cmp(png->ipbuf, 0, DATASIZE)) {
             MSG_WARN("\"%s\" is not a PNG file.\n", URL_STR(png->url));
             png->state = IS_finished;
             break;