i#1696 x64 underflow: fix 64-bit memory corruption
i#1696 x64 underflow: fix 64-bit memory corruption

Fixes a bug where reg_shadow_addr() on 64-bit ends up with a negative
displacement, corrupting DR's TLS memory.

Fixes #1696

Review-URL: https://codereview.appspot.com/216930043

code4bones/drmemory,sigma-random/drmemory,sigma-random/drmemory,sigma-random/drmemory,code4bones/drmemory,code4bones/drmemory,LohithBlaze/drmemory,LohithBlaze/drmemory,sigma-random/drmemory,LohithBlaze/drmemory,code4bones/drmemory,LohithBlaze/drmemory,LohithBlaze/drmemory,code4bones/drmemory,sigma-random/drmemory