Only clear the minsym array when necessary

Only clear the minsym array when necessary

The array starts out initialized to zero:
  minimal_symbol *msymbol_hash[MINIMAL_SYMBOL_HASH_SIZE] {};

So we only need to explicitly clear it if there were previous minsyms
added to it. This patch does that.

gdb/ChangeLog:

2019-10-30  Christian Biesinger  <cbiesinger@google.com>

	* minsyms.c (clear_minimal_symbol_hash_tables): New function.
	(build_minimal_symbol_hash_tables): Code to clear the table moved
	to clear_minimal_symbol_hash_tables.
	(minimal_symbol_reader::install): Call clear_minimal_symbol_hash_tables
	when needed.

Change-Id: I7da994fe6747f67714e7efe9fdbb0dbc4d6ea532