[aarch64] Allocate space for err_str in aarch64_handle_attr_branch_protection

[aarch64] Allocate space for err_str in aarch64_handle_attr_branch_protection

-fsanitize=hwaddress found a one-byte overwrite when running the
testsuite here.  aarch64_handle_attr_branch_protection allocates
`strlen(str)` bytes for an error string, which is populated by
`strcpy(..., str)` in the case where the branch protection string is
completely invalid.

Not tested -- I don't want to re-build and it seems obvious.

gcc/ChangeLog:

2019-11-05  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/aarch64/aarch64.c (aarch64_handle_attr_cpu): Allocate
	enough bytes for the NULL character.

From-SVN: r277845