Fix compile time warning message for the AArch64 BFD backend, about a possible attemp...

Fix compile time warning message for the AArch64 BFD backend, about a possible attempt to call sprintf on a NULL buffer pointer.

	* elfnn-aarch64.c (_bfd_aarch64_erratum_835769_stub_name): Check
	for malloc returning NULL.
	(_bfd_aarch64_erratum_843419_fixup): Check for
	_bfd_aarch64_erratum_835769_stub_name returning NULL.