[libpng16] Safely convert num_bytes to a png_byte in png_set_sig_bytes()

ANNOUNCE

@@ -55,6 +55,8 @@ Version 1.6.19beta02 [August 18, 2015]
     on a 16-bit system. It also adds back various switch default clauses
     for GCC; GCC errors out if they are not present (with an appropriately
     high level of warnings).
+  Safely convert num_bytes to a png_byte in png_set_sig_bytes() (Robert
+    Seacord).
 
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CHANGES

@@ -5335,6 +5335,8 @@ Version 1.6.19beta02 [August 18, 2015]
     on a 16-bit system. It also adds back various switch default clauses
     for GCC; GCC errors out if they are not present (with an appropriately
     high level of warnings).
+  Safely convert num_bytes to a png_byte in png_set_sig_bytes() (Robert
+    Seacord).
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit

png.c

@@ -26,15 +26,17 @@ typedef png_libpng_version_1_6_19beta02 Your_png_h_is_not_version_1_6_19beta02;
 void PNGAPI
 png_set_sig_bytes(png_structrp png_ptr, int num_bytes)
 {
+   unsigned int nb = (unsigned int)num_bytes;
+
    png_debug(1, "in png_set_sig_bytes");
 
    if (png_ptr == NULL)
       return;
 
-   if (num_bytes > 8)
+   if (nb > 8)
       png_error(png_ptr, "Too many bytes for PNG signature");
 
-   png_ptr->sig_bytes = (png_byte)((num_bytes < 0 ? 0 : num_bytes) & 0xff);
+   png_ptr->sig_bytes = (png_byte)nb;
 }
 
 /* Checks whether the supplied bytes match the PNG signature.  We allow