Fix segfaults after callbacks originating in JIT compiled calls to na…

…tive code

Before running a non-toplevel frame, we need to make sure, the caller's frame
has a valid return_address. While MVM_nativecall_invoke_jit and
MVM_nativecall_invoke did so, the JIT implementation of calls to JIT compiled
native functions did not. This could lead to segfaults when native code called
from JIT compiled code ran callbacks which then tried to return to the NULL
address.

Fix by initializing tc->cur_frame->return_address in the callback_handler, so
it doesn't matter how we ended up there.

src/core/nativecall.c

@@ -1265,8 +1265,6 @@ void MVM_nativecall_invoke_jit(MVMThreadContext *tc, MVMObject *site) {
         body = MVM_nativecall_get_nc_body(tc, site);
     }
 
-    tc->cur_frame->return_address = *tc->interp_cur_op;
-
     MVMJitCode * const jitcode = body->jitcode;
     jitcode->func_ptr(tc, *tc->interp_cu, jitcode->labels[0]);
 }

src/core/nativecall_dyncall.c

@@ -342,6 +342,7 @@ static char callback_handler(DCCallback *cb, DCArgs *cb_args, DCValue *cb_result
 
             tc->cur_frame->return_value = &res;
             tc->cur_frame->return_type  = MVM_RETURN_OBJ;
+            tc->cur_frame->return_address = *tc->interp_cur_op;
 
             tc->nested_interpreter++;
             MVM_interp_run(tc, callback_invoke, &cid);
@@ -504,8 +505,6 @@ MVMObject * MVM_nativecall_invoke(MVMThreadContext *tc, MVMObject *res_type,
     unsigned int interval_id;
     DCCallVM *vm;
 
-    tc->cur_frame->return_address = *tc->interp_cur_op;
-
     /* Create and set up call VM. */
     vm = dcNewCallVM(8192);
     dcMode(vm, body->convention);