[Process] Fix WriteMemory return value

Summary:
In case of a breakpoint site overlapping with the destination address,
the WriteMemory method reported an incorrect memory size.

Instead of returning the right amount of bytes written, it falls through
the scope and returned 0.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

Reviewers: jasonmolenda, friss, jingham

Subscribers: JDevlieghere, davide, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60022

llvm-svn: 357420