std: check for overflow in writeCurrentStackTrace
std: check for overflow in writeCurrentStackTrace

On arm64 macOS, the address of the last frame is 0x0 rather than
a positive value like 0x1 on x86_64 macOS, therefore, we overflow
an integer trying to subtract 1 when printing the stack trace. This
patch fixes it by first checking for this condition before trying
to subtract 1.

Note that we do not need to signal the `SignalIterator` about this
as it will correctly detect this condition on the subsequent iteration
and return `null`, thus terminating the loop.

zig-lang/zig,andrewrk/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig