scripts: Fix the used sed commands at the end of retdec-decompiler.sh…

… to work on macOS (#257).

The original commands failed with the following error on macOS:

    sed: 1: "/private/tmp/test.c": undefined label 'mp/test.c'
    sed: 1: "/private/tmp/test.c": undefined label 'mp/test.c'

The reason why the commands failed is that when using the -i parameter of sed
on macOS, you need to specify an extension of the backup file. If you do not
want to create a backup file, you have to pass the empty string. On Linux,
using just -i without any extension suffices.

The new sed command should work on Linux, macOS, and Windows, without a need to
install gnu-sed on macOS.

scripts/retdec-decompiler.sh

@@ -953,8 +953,9 @@ fi
 # Remove trailing whitespace and the last redundant empty new line from the
 # generated output (if any). It is difficult to do this in the back-end, so we
 # do it here.
-sed -i 's/[ \t]*$//' "$OUT"
-sed -i '$ { /^$/ d}' "$OUT"
+# Note: When modifying the command below, ensure that it works also on macOS as
+#       its sed differs from the GNU sed.
+sed -e :a -e '/^\n*$/{$d;N;};/\n$/ba' -e 's/[[:space:]]*$//' -i'' -- "$OUT"
 
 # Colorize output file.
 if [ "$COLOR_IDA" ]; then