[lldb] Don't send invalid region addresses to lldb server

Previously when <addr> in "memory region <addr>" didn't
parse correctly, we'd print an error then also ask lldb-server
for a region containing LLDB_INVALID_ADDRESS.

(lldb) memory region not_an_address
error: invalid address argument "not_an_address"...
error: Server returned invalid range

Only send the command to lldb-server if the address
parsed correctly.

(lldb) memory region not_an_address
error: invalid address argument "not_an_address"...

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D87694