[JITLink][MachO][arm64] Fix error-check order.

The error check should be performed after the iterator increment, not before
it. Thanks to @dcb314 for catching this!

Fixes github.com/swiftlang/swift/issues/81119

Author: lhames

llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp

@@ -312,10 +312,10 @@ class MachOLinkGraphBuilder_arm64 : public MachOLinkGraphBuilder {
 
           Addend = SignExtend64(RI.r_symbolnum, 24);
 
+          ++RelItr;
           if (RelItr == RelEnd)
             return make_error<JITLinkError>("Unpaired Addend reloc at " +
                                             formatv("{0:x16}", FixupAddress));
-          ++RelItr;
           RI = getRelocationInfo(RelItr);
 
           MachORelocKind = getRelocationKind(RI);