Fix a segfault.

this patch fixes a rare but reproducible segfault observed playing a
multi-threaded match, it is discussed somewhat in fishcooking.

From the core file, it could be observed that the issue was in qsearch, namely:

````
   ss->pv[0] = MOVE_NONE;
````

and the backtrace shows the it arrives there via razoring, called from the rootNode:

````
    (gdb) bt
    alpha=-19, beta=682, depth=DEPTH_ZERO) at search.cpp:1247
````

Indeed, ss->pv can indeed by a nullptr at the rootNode. However, why is the
segfault so rare ?

The reason is that the condition that guards razoring:

````
   (depth < 2 * ONE_PLY &&  eval <= alpha - RazorMargin)
````

is almost never true, since at the root alpha for depth < 5 is -VALUE_INFINITE.

Nevertheless with the new failHigh scheme, this is not guaranteed, and rootDepth > 5,
can still result in a depth < 2 search at the rootNode. If now another thread,
via the hash, writes a new low eval to the rootPos qsearch can be entered.
Rare but not unseen... I assume that some of the crashes in fishtest recently
might be due to this.

Closes #1860

No functional change

src/search.cpp

@@ -303,6 +303,7 @@ void MainThread::search() {
 void Thread::search() {
 
   Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
+  Move  pv[MAX_PLY+1];
   Value bestValue, alpha, beta, delta;
   Move  lastBestMove = MOVE_NONE;
   Depth lastBestMoveDepth = DEPTH_ZERO;
@@ -314,6 +315,7 @@ void Thread::search() {
   std::memset(ss-4, 0, 7 * sizeof(Stack));
   for (int i = 4; i > 0; i--)
      (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
+  ss->pv = pv;
 
   bestValue = delta = alpha = -VALUE_INFINITE;
   beta = VALUE_INFINITE;
@@ -756,8 +758,9 @@ namespace {
     }
 
     // Step 7. Razoring (~2 Elo)
-    if (   depth < 2 * ONE_PLY
-        && eval <= alpha - RazorMargin)
+    if (   !rootNode // The required rootNode PV handling is not available in qsearch
+        &&  depth < 2 * ONE_PLY
+        &&  eval <= alpha - RazorMargin)
         return qsearch<NT>(pos, ss, alpha, beta);
 
     improving =   ss->staticEval >= (ss-2)->staticEval