CVE-2018-6340 Fix buffer overread

Summary:
snprintf() returns the number of would be written bytes if the buffer had
enough size, letting the original code to construct output string from
arbitrary data on the stack.

Fix this by properly assembling the String without trying to arbitrarily
limiting the length to 30 bytes.

Reviewed By: ottoni

Differential Revision: D13509547

fbshipit-source-id: aa401b83d356f5b261433c4b16d777948ba7f9b4