Fixed gdb-remote M test length parameter, added overwrite to test.

Initial check-in provided a nibble count instead of byte count for
the memory to write.  Fixed that.

Enhanced test to check for overwrite past the expected range of
writing to verify the correct amount is written.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@210602 91177308-0d34-0410-b5e6-96231b3b80d8