libc/yp: fix memory issues and compiler warnings in xdryp.c

[aalx2176_gmail.com]

Summary

• Move bzero() inside the while loop
• Add malloc() u_int overflow check
• Fix use-after-free by moving xdr_free() to cleanup block
• Fix warnings in xdr_free() by casting function pointers to (void *)
• Add missing headers and forward declarations

Test Plan

Compiled with -Weverything -Wno-unsafe-buffer-usage -Wno-sign-conversion