Zero DDS header reserved2 field (#654)

When writing uncompressed DDS files the current code does not initialize
the reserved2 field, so it contains random stack data. This change zeros 
reserved2 before writing it.

Author: sahvx655-wq

Docs/ChangeLog-5x.md

@@ -14,6 +14,9 @@ clocked at 4.2 GHz, running `astcenc` using AVX2 and 6 threads.
 The 5.6.0 release is a minor maintenance release.
 
 * **Command line tool updates:**
+  * **Bug fix:** Zero the trailing reserved field of the DDS header before
+    writing it, so uncompressed `.dds` output no longer contains uninitialized
+    stack bytes.
   * **Bug fix:** Fixed incorrect plane stride when writing an uncompressed 3D
     LDR image to a DDS container.
   * **Bug fix:** Fixed potential integer overflow when storing very large

Source/astcenccli_image_load_store.cpp

@@ -2249,6 +2249,7 @@ static bool store_dds_uncompressed_image(
 	hdr.caps2 = (dim_z > 1) ? 0x200000 : 0;
 	hdr.caps3 = 0;
 	hdr.caps4 = 0;
+	hdr.reserved2 = 0;
 
 	// Pixel-format data
 	if (bitness == 8)