remote: return EINVALIDSPEC when given an empty URL

This is what we used to return in the settter and there's tests in
bindings which ask for this. There's no particular reason to stop doing
so.

src/remote.c

@@ -167,14 +167,18 @@ static int get_check_cert(int *out, git_repository *repo)
 
 static int canonicalize_url(git_buf *out, const char *in)
 {
-#ifdef GIT_WIN32
-	const char *c;
+	if (in == NULL || strlen(in) == 0) {
+		giterr_set(GITERR_INVALID, "cannot set empty URL");
+		return GIT_EINVALIDSPEC;
+	}
 
+#ifdef GIT_WIN32
 	/* Given a UNC path like \\server\path, we need to convert this
 	 * to //server/path for compatibility with core git.
 	 */
 	if (in[0] == '\\' && in[1] == '\\' &&
 		(git__isalpha(in[2]) || git__isdigit(in[2]))) {
+		const char *c;
 		for (c = in; *c; c++)
 			git_buf_putc(out, *c == '\\' ? '/' : *c);