shared.c: Only setenv() if value is non-null

Some setenv() implementations (e.g. the one in OpenBSD's stdlib)
segfault if we pass a NULL value. Only set environment variables if the
corresponding settings are defined to avoid this.

Note that this is a minor behaviour change as environment variables were
supposed to be set to an empty string if a setting was undefined. Given
that this feature isn't part of any official release yet, there's no
need to worry about backwards compatibility, really. Change the
documentation accordingly.

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

shared.c

@@ -392,7 +392,7 @@ void cgit_prepare_repo_env(struct cgit_repo * repo)
 	p = env_vars;
 	q = p + env_var_count;
 	for (; p < q; p++)
-		if (setenv(p->name, p->value, 1))
+		if (p->value && setenv(p->name, p->value, 1))
 			fprintf(stderr, warn, p->name, p->value);
 }