Fix jl_new_task to avoid allocating stack twice

If the user requested dedicated stack of a certain size, then `jl_new_task`
immediately allocates the stack by invoking `jl_alloc_fiber`.

However, it also later nulls `t->stkbuf`. As a result, `ctx_switch` invokes
`jl_alloc_fiber` *again* to allocate another stack.

src/task.c

@@ -676,6 +676,7 @@ JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion
         else {
             t->bufsz = JL_STACK_SIZE;
         }
+        t->stkbuf = NULL;
     }
     else {
         // user requested dedicated stack of a certain size
@@ -701,7 +702,6 @@ JL_DLLEXPORT jl_task_t *jl_new_task(jl_function_t *start, jl_value_t *completion
     t->sticky = 1;
     t->gcstack = NULL;
     t->excstack = NULL;
-    t->stkbuf = NULL;
     t->started = 0;
     t->prio = -1;
     t->tid = -1;