Remove always-false null checks

There will always be at least one callstack record below an executing
frame record, and if that is a region start, it means there's a whole
region of records prior to it. Spotted by mlschroe++.

src/core/args.c

@@ -1081,7 +1081,7 @@ void MVM_args_bind_failed(MVMThreadContext *tc, MVMDispInlineCacheEntry **ice_pt
      * bind failure handler. This is determined by if there is a bind failure
      * frame under us on the callstack in a fresh state. */
     MVMCallStackRecord *under_us = tc->stack_top->prev;
-    while (under_us && under_us->kind == MVM_CALLSTACK_RECORD_START_REGION)
+    while (under_us->kind == MVM_CALLSTACK_RECORD_START_REGION)
         under_us = under_us->prev;
     if (under_us->kind == MVM_CALLSTACK_RECORD_BIND_CONTROL) {
         MVMCallStackBindControl *control_record = (MVMCallStackBindControl *)under_us;
@@ -1119,7 +1119,7 @@ void MVM_args_bind_failed(MVMThreadContext *tc, MVMDispInlineCacheEntry **ice_pt
  * dispatch resumption. */
 void MVM_args_bind_succeeded(MVMThreadContext *tc, MVMDispInlineCacheEntry **ice_ptr) {
     MVMCallStackRecord *under_us = tc->stack_top->prev;
-    while (under_us && under_us->kind == MVM_CALLSTACK_RECORD_START_REGION)
+    while (under_us->kind == MVM_CALLSTACK_RECORD_START_REGION)
         under_us = under_us->prev;
     if (under_us->kind == MVM_CALLSTACK_RECORD_BIND_CONTROL) {
         MVMCallStackBindControl *control_record = (MVMCallStackBindControl *)under_us;