libunwind: Don't attempt to authenticate a null return address.

Null return addresses can appear at the bottom of the stack (i.e. the
frame corresponding to the entry point). Authenticating these addresses
will set the error code in the address, which will lead to a segfault
in the sigreturn trampoline detection code. Fix this problem by not
authenticating null addresses.

Differential Revision:

GitOrigin-RevId: cddc53ef088b68586094c9841a76b41bee3994a4
diff --git a/src/DwarfInstructions.hpp b/src/DwarfInstructions.hpp
index c39cabe..bd13d16 100644
--- a/src/DwarfInstructions.hpp
+++ b/src/DwarfInstructions.hpp
@@ -213,7 +213,8 @@
       // restored. autia1716 is used instead of autia as autia1716 assembles
       // to a NOP on pre-v8.3a architectures.
       if ((R::getArch() == REGISTERS_ARM64) &&
-          prolog.savedRegisters[UNW_ARM64_RA_SIGN_STATE].value) {
+          prolog.savedRegisters[UNW_ARM64_RA_SIGN_STATE].value &&
+          returnAddress != 0) {
         return UNW_ECROSSRASIGNING;