fix if test for file name with spaces

 fontpreview | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fontpreview b/fontpreview
index 1c66567..1dbd993 100755
--- a/fontpreview
+++ b/fontpreview
@@ -185,8 +185,8 @@ while true; do
     shift
 done
 
-if [[ $input_file != "" ]] ; then
-    if [ -z $output_file ] ; then
+if [[ "$input_file" != "" ]] ; then
+    if [ -z "$output_file" ] ; then
         output_file="${input_file}.png"
     fi
     generate_preview "$input_file" "$output_file"