version.sh: fix empty result when git describe fails

fixes an error in travis, which makes a shallow clone of 50 commits.
if the last tag is older than 50 commits, we get:
"fatal: No names found, cannot describe anything."

this caused a premature exit due to an assert error in safe_write()
on this line: assert (count > 0);

because the version variable in tinyproxy was empty.

scripts/version.sh

@@ -5,8 +5,12 @@ GIT_DIR="${SCRIPT_DIR}/../.git"
 
 if test -d "${GIT_DIR}" ; then
 	if type git >/dev/null 2>&1 ; then
-		git describe --match '[0-9]*.[0-9]*.[0-9]*' 2>/dev/null \
-			| sed -e 's/-/-git-/'
+		gitstr=$(git describe --match '[0-9]*.[0-9]*.[0-9]*' 2>/dev/null)
+		if test "x$?" != x0 ; then
+			sed 's/$/-git/' < VERSION
+		else
+			printf "%s\n" "$gitstr" | sed -e 's/-/-git-/'
+		fi
 	else
 		sed 's/$/-git/' < VERSION
 	fi