Bug fix: Random checksum index could sneak in via boot catalog node

and cause a SIGSEGV if checksumming is enabled.
This and other occurences of malloc() were changed to calloc().

libisofs/filesrc.c

@@ -556,7 +556,7 @@ int iso_file_src_writer_create(Ecma119Image *target)
 {
     IsoImageWriter *writer;
 
-    writer = malloc(sizeof(IsoImageWriter));
+    writer = calloc(1, sizeof(IsoImageWriter));
     if (writer == NULL) {
         return ISO_OUT_OF_MEM;
     }