AppRun fixed libva-x11 not being LD_PRELOADED

In the second iteration of for cycle, $NAME was derived from something
like "/usr/lib/libva.so.2:/usr/lib/libva-drm.so.2"  (s/libva/libva-x11),
which obviously didn't exist as a file.

Fixes:

    $ <AppImage> --list-modules | grep -A 2 'Errors:'
    Errors:
            ultragrid_acompress_libavcodec.so
                    ./squashfs-root2/usr/lib/libva-x11.so.2: undefined symbol: va_fool_postp

data/scripts/Linux-AppImage/AppRun

@@ -129,14 +129,15 @@ set_libva_ld_preload() {
         if [ -z "$LIBVA_LIB" ]; then
                 return 1
         fi
+        libva_libs=$LIBVA_LIB
         # add also libva-drm, libva-x11 if present
         for n in libva-drm libva-x11; do
                 NAME=$(echo "$LIBVA_LIB" | sed s/libva/$n/)
                 if [ -f "$NAME" ]; then
-                        LIBVA_LIB=$LIBVA_LIB:$NAME
+                        libva_libs=$libva_libs:$NAME
                 fi
         done
-        export LD_PRELOAD="$LIBVA_LIB${LD_PRELOAD:+:$LD_PRELOAD}"
+        export LD_PRELOAD="$libva_libs${LD_PRELOAD:+:$LD_PRELOAD}"
         return 0
 }