jpeg: Do not rely on UB around setjmp/longjmp

We are reading and writing a structure before and after a sigsetjmp/longjmp pair without declaring it volatile. This is undefined behaviour, and even if most compilers and toolchains won't have any issue with that, it's better to avoid it if at all possible.

The simplest fix is to declare the variable in a separate function, and then pass it by reference.

Fixes: #143 (closed)