xzgrep: Make the fix for ZDI-CAN-16587 more robust.

I don't know if this can make a difference in the real world
but it looked kind of suspicious (what happens with sed
implementations that cannot process very long lines?).
At least this commit shouldn't make it worse.

src/scripts/xzgrep.in

@@ -223,7 +223,10 @@ for i; do
       case $i in
       (*'
 '* | *'&'* | *'\'* | *'|'*)
-        i=$(printf '%s\n' "$i" | LC_ALL=C sed 's/[&\|]/\\&/g; $!s/$/\\/');;
+        # If sed fails, set i to a known safe string to ensure that
+        # failing sed didn't create a half-escaped dangerous string.
+        i=$(printf '%s\n' "$i" | LC_ALL=C sed 's/[&\|]/\\&/g; $!s/$/\\/') ||
+            i='(unknown filename):';;
       esac
 
       # $i already ends with a colon so don't add it here.