Fixing NULL terminator for test cert.

Forgot to do that when I replaced the cert in #4096. Thanks asan for
catching that!

test/core/end2end/data/server1_cert.c

@@ -112,4 +112,4 @@ const char test_server1_cert[] = {
     0x76, 0x31, 0x4c, 0x4e, 0x49, 0x52, 0x33, 0x6c, 0x6f, 0x76, 0x65, 0x55,
     0x38, 0x0a, 0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x45, 0x4e, 0x44, 0x20, 0x43,
     0x45, 0x52, 0x54, 0x49, 0x46, 0x49, 0x43, 0x41, 0x54, 0x45, 0x2d, 0x2d,
-    0x2d, 0x2d, 0x2d, 0x0a};
+    0x2d, 0x2d, 0x2d, 0x0a, 0x00};