[vm/profiler] Handle out of blocks case in ReserveSampleAndLink.

Both SampleBlock::ReserveAllocationSample and
SampleBlock::ReserveCPUSample can return nullptr, so that needs to be
appropriately handled in SampleBlock::ReserveSampleAndLink. The only
caller of the latter checks for nullptr, so pass though nullptr results
from the former methods.

TEST=Fixes current service tests failures on some trybots,
     see test results on CL.

Cq-Include-Trybots: luci.dart.try:vm-kernel-linux-debug-ia32-try,app-kernel-linux-debug-x64-try
Change-Id: Ib70cc6726862bc5fe3bfb0c06ee2aae84ec026c3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208321
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Clement Skau <cskau@google.com>

runtime/vm/profiler.cc

@@ -282,6 +282,9 @@ Sample* SampleBlock::ReserveSampleAndLink(Sample* previous) {
   Sample* next = previous->is_allocation_sample()
                      ? buffer->ReserveAllocationSample(isolate)
                      : buffer->ReserveCPUSample(isolate);
+  if (next == nullptr) {
+    return nullptr;  // No blocks left, so drop sample.
+  }
   next->Init(previous->port(), previous->timestamp(), previous->tid());
   next->set_head_sample(false);
   // Mark that previous continues at next.