Only make a nullterminated string if we need to

Only make a nullterminated string if we need to

As of 7bb43059820c5febb4509b15202a93efde442bc6, we no longer need
a nullterminated linkage_name to look up the entry in the hash table.

So this patch makes it so we only make the copy if the entry was
not found.

By auditing all callers of symbol_set_names, I found out that all cases
where the string may not be nullterminated already pass true for COPY_NAME.
So here, I am documenting that as a requirement and am removing the code
that relies on undefined behavior in symbol_set_names (it accessed the string
past the provided length to check for nulltermination). Note that the Ada
case at the beginning of symbol_set_names was already relying on this.

gdb/ChangeLog:

2019-10-29  Christian Biesinger  <cbiesinger@google.com>

	* symtab.h (symbol_set_names): Document that copy_name must be
	set to true for non-nullterminated strings.
	* symtab.c (symbol_set_names): Only make a nullterminated copy of
	linkage_name if the entry was not found and we need to demangle.

Change-Id: I183302e1f51483ff6dff0fd5c3b0f32f0f04a5d2