modify_ldt: initialize scratch

We store a pointer in the scratch area, but only conditionally. Hence
there can be an uninitialized value which causes problems in the
tear-down path:
*** Error in `trinity': double free or corruption (!prev):
0x0000000002484530 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x740af)[0x7f2a66eb60af]
/lib64/libc.so.6(+0x798de)[0x7f2a66ebb8de]
/lib64/libc.so.6(+0x7a5e6)[0x7f2a66ebc5e6]
trinity[0x40abc8]
trinity[0x40dabc]
trinity[0x40338a]
trinity[0x40704c]
trinity[0x402c47]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7f2a66e63b15]
trinity[0x402f13]