cgit.c: Do not restore unset environment variables

getenv() returns a NULL pointer if the specified variable name cannot be
found in the environment. However, some setenv() implementations crash
if a NULL pointer is passed as second argument. Only restore variables
that are not NULL.

See commit d96d2c98eb for a related patch.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

cgit.c

@@ -486,8 +486,10 @@ static int prepare_repo_cmd(struct cgit_context *ctx)
 	init_display_notes(NULL);
 
 	/* We restore the unset variables afterward. */
-	setenv("HOME", user_home, 1);
-	setenv("XDG_CONFIG_HOME", xdg_home, 1);
+	if (user_home)
+		setenv("HOME", user_home, 1);
+	if (xdg_home)
+		setenv("XDG_CONFIG_HOME", xdg_home, 1);
 
 	if (nongit) {
 		const char *name = ctx->repo->name;