Fix segfault in AArch64 backend with -g and -mbig-endian

Fix a null pointer dereference when trying to swap the endianness of
fixups in the .eh_frame section in the AArch64 backend.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218311 91177308-0d34-0410-b5e6-96231b3b80d8