Fix regression caused by minimal symbol changes

Fix regression caused by minimal symbol changes

The earlier patch to change minimal symbol allocations to use xmalloc
erroneously left a call to obstack_blank in
minimal_symbol_reader::install.  Because obstack_blank does not finish
the object allocation on an obstack, this in turn could cause invalid
memory reads in some situations.

This patch fixes the problem by removing the call.  Tested on x86-64
Fedora 29; also verified with valgrind.

gdb/ChangeLog
2019-03-18  Tom Tromey  <tromey@adacore.com>

	* minsyms.c (minimal_symbol_reader::install): Remove call to
	obstack_blank.