[weex_core] fix guess crash point (#1856)

Fix following crash

> signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 57286b7f
  r0 89a8e0c0  r1 ff86f5ec  r2 89a8e0c0  r3 57286b63
  r4 89a8e0c0  r5 12e23560  r6 c3298dcc  r7 6fba3af4
  r8 147c9780  r9 eb505400  10 00000000  fp 00000000
  ip a6af6dbc  sp ff86f5c8  lr a6a5ec5f  pc a6a1d4ec  cpsr 800f0030
    #00 pc 0x7d4ec libweexcore.so (_ZN8WeexCore16WXCoreLayoutNode30getLayoutDirectionFromPathNodeEv+5)
    #1 pc 0xbec5b libweexcore.so
    #2 pc 0xb751ed base.odex

Author: win80540

weex_core/Source/core/layout/layout.cpp

@@ -1120,13 +1120,16 @@ namespace WeexCore {
 
     WXCoreDirection WXCoreLayoutNode::getLayoutDirectionFromPathNode() {
         WXCoreLayoutNode *node = this;
-        if (node->getLayoutDirection() != kDirectionInherit) return node->getLayoutDirection();
-        if (node->getDirection() != kDirectionInherit) {
-            node->mLayoutResult->mLayoutDirection = node->getDirection();
-            return node->getLayoutDirection();
-        } else if (nullptr != node->mParent) {
-            node->mLayoutResult->mLayoutDirection = node->mParent->getLayoutDirectionFromPathNode();
-            return node->getLayoutDirection();
+        if (nullptr != node) {
+            if (node->getLayoutDirection() != kDirectionInherit)
+                return node->getLayoutDirection();
+            if (node->getDirection() != kDirectionInherit) {
+                node->mLayoutResult->mLayoutDirection = node->getDirection();
+                return node->getLayoutDirection();
+            } else if (nullptr != node->mParent) {
+                node->mLayoutResult->mLayoutDirection = node->mParent->getLayoutDirectionFromPathNode();
+                return node->getLayoutDirection();
+            }
         }
         return WEEXCORE_CSS_DEFAULT_DIRECTION;
     }