Set null-terminator after strncpy

Summary: When the length of `pkcs12.second.c_str()` is equal to `length`, strncpy won't write a null-terminator at the end of the destination string, making the destination unsafe to read and potentially cause an overflow.

Reviewed By: lblasa

Differential Revision: D46934320

fbshipit-source-id: 5e7acd49523b80105bcc47471facd9ff23b8a2b8

iOS/FlipperKit/FlipperWebSocket.mm

@@ -92,6 +92,7 @@
         return std::string("");
       }
       strncpy(password, pkcs12.second.c_str(), length);
+      password[length - 1] = '\0';
       return pkcs12.first;
     };
   }