Use unique_xmalloc_ptr in two solib functions

This removes a couple of cleanups by using unique_xmalloc_ptr instead.
These two changes are combined because the two functions are very
similar.

gdb/ChangeLog
2018-06-18  Tom Tromey  <tom@tromey.com>

	* solib-frv.c (frv_relocate_main_executable): Use
	unique_xmalloc_ptr.
	* solib-dsbt.c (dsbt_relocate_main_executable): Use
	unique_xmalloc_ptr.