Fix some null pointer access crashes

src/cssize.cpp

@@ -380,7 +380,7 @@ namespace Sass {
 
   bool Cssize::bubblable(Statement* s)
   {
-    return Cast<StyleRule>(s) || s->bubbles();
+    return Cast<StyleRule>(s) || (s && s->bubbles());
   }
 
   Block* Cssize::flatten(const Block* b)
@@ -479,7 +479,8 @@ namespace Sass {
                                     children->pstate(),
                                     children->length(),
                                     children->is_root());
-        bb->append(ss->perform(this));
+        auto evaled = ss->perform(this);
+        if (evaled) bb->append(evaled);
 
         Block_Obj wrapper_block = SASS_MEMORY_NEW(Block,
                                               children->pstate(),

src/extender.cpp

@@ -147,6 +147,7 @@ namespace Sass {
     for (auto target : extensions) {
       SimpleSelector* key = target.first;
       ExtSelExtMapEntry& val = target.second;
+      if (val.empty()) continue;
       if (originals.find(key) == originals.end()) {
         const Extension& extension = val.front().second;
         if (extension.isOptional) continue;

src/memory/memory_pool.hpp

@@ -55,7 +55,9 @@ namespace Sass {
     std::vector<void*> arenas;
 
     // One pointer for every bucket (zero init)
+    #ifdef _MSC_VER
     #pragma warning (suppress:4351)
+    #endif
     void* freeList[SassAllocatorBuckets]{};
 
     // Increase the address until it sits on a

src/output.cpp

@@ -289,7 +289,7 @@ namespace Sass {
 
     for (size_t i = 0, L = b->length(); i < L; ++i) {
       Statement_Obj stm = b->get(i);
-      stm->perform(this);
+      if (stm) stm->perform(this);
       if (i < L - 1 && format) append_special_linefeed();
     }