Use vm_commit

[sogaiu]

This PR contains a suggestion to use vm_commit(); in place of janet_stack_frame(stack)->pc = pc;.

The vm_commit(); construct is used elsewhere within the JOP_CALL handling in which this instance of janet_stack_frame(stack)->pc = pc; appears:

• https://github.com/janet-lang/janet/blob/3a4d56afca2eee0e4dbca9a6400757d198e6d6f3/src/core/vm.c#L975
• https://github.com/janet-lang/janet/blob/3a4d56afca2eee0e4dbca9a6400757d198e6d6f3/src/core/vm.c#L993
• https://github.com/janet-lang/janet/blob/3a4d56afca2eee0e4dbca9a6400757d198e6d6f3/src/core/vm.c#L1002

Perhaps it makes sense to use vm_commit(); on line 983 as well?

Though I can see that if one wanted to leave the JANET_FUNCTION-handling parts of JOP_CALL and JOP_TAILCALL easier to compare, perhaps not using vm_commit(); might be preferrable.

[bakpakin]

LGTM