Fix CVE-2021-32490: out of bounds write
https://sourceforge.net/p/djvu/djvulibre-git/ci/cd8b5c97b27a5c1dc83046498b6ca49ad20aa9b6

Index: libdjvu/IW44Image.cpp
--- libdjvu/IW44Image.cpp.orig
+++ libdjvu/IW44Image.cpp
@@ -676,9 +676,13 @@ IW44Image::Map::image(signed char *img8, int rowsize, 
   // Allocate reconstruction buffer
   short *data16;
   size_t sz = bw * bh;
+  if (sz == 0)
+    G_THROW("IW44Image: image size is zero (corrupted file?)");
   if (sz / (size_t)bw != (size_t)bh) // multiplication overflow
     G_THROW("IW44Image: image size exceeds maximum (corrupted file?)");
   GPBuffer<short> gdata16(data16,sz);
+  if (data16 == 0)
+    G_THROW("IW44Image: unable to allocate image buffer");
   // Copy coefficients
   int i;
   short *p = data16;
