Fixes a regression from the #1660 (closed) fix which changes the executable_path buffer after most initialization, yet fails to update the executable_basename pointer into that same buffer.
Adds a test by changing the linux.execve64 test to use a relative path for the sub-path, and having that client check for slashes in dr_get_application_name(). Confirmed the new test fails without this fix.
Issue: #1660 (closed), #4892 (closed) Fixes #4892 (closed)