Fix computed goto into statement expression (undefined behavior)

[guillerodriguez]

The DEF_OPC_JMP macros in interp-inlining.h place BRANCH labels inside GCC statement expressions ({ }). These labels are targets of computed gotos via the interpreter dispatch table. Per GCC documentation, jumping into a statement expression with a computed goto is undefined behavior 1, so this is fragile and not guaranteed to work.

Although GCC does detect this in some simple cases, it does not catch it in JamVM, possibly due to the way the code is structured (label addresses are collected into the dispatch table separately from the goto *pc->handler dispatch in the interpreter loop). Clang 17+ performs whole-function analysis of indirect goto targets and rejects the code with a hard error:

error: cannot jump from this indirect goto statement to one of
its possible targets
note: possible target of indirect goto statement
note: jump enters a statement expression

Only interp-inlining.h is affected. The other interpreter variants (interp-direct.h, interp-indirect.h) do not create labels inside statement expressions in their BRANCH macros.

Fix by changing ({ }) to { } in DEF_OPC_JMP. The statement expression return value was never used (BODY is in statement context and always exits via goto *pc->handler), so the change is purely syntactic. Double checked that GCC produces byte-identical machine code with and without this change for all optimization levels.

[guillerodriguez]

From GCC documentation:

Jumping into a statement expression with goto or using a switch statement outside the statement expression with a case or default label inside the statement expression is not permitted. Jumping into a statement expression with a computed goto (see Labels as Values) has undefined behavior.