connect.c: guard config parser from value=NULL

core.gitproxy configuration expects a string value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

connect.c

diff --git a/connect.c b/connect.c
index 3aefd4a..71597d4 100644
--- a/connect.c
+++ b/connect.c
@@ -370,6 +370,8 @@
 
 		if (git_proxy_command)
 			return 0;
+		if (!value)
+			return config_error_nonbool(var);
 		/* [core]
 		 * ;# matches www.kernel.org as well
 		 * gitproxy = netcatter-1 for kernel.org