Merge pull request #13 from Lastique/fix_advancing_nullptr

Avoid advancing null pointers, which is UB

include/boost/xpressive/detail/utility/sequence_stack.hpp

@@ -15,6 +15,7 @@
 # pragma warning(disable : 4127) // conditional expression constant
 #endif
 
+#include <cstddef>
 #include <algorithm>
 #include <functional>
 
@@ -210,22 +211,20 @@ public:
 
     T *push_sequence(std::size_t count, T const &t)
     {
-        // This is the ptr to return
-        T *ptr = this->curr_;
-
-        // Advance the high-water mark
-        this->curr_ += count;
-
         // Check to see if we have overflowed this buffer
-        if(std::less<void*>()(this->end_, this->curr_))
+        std::size_t size_left = static_cast< std::size_t >(this->end_ - this->curr_);
+        if (size_left < count)
         {
-            // oops, back this out.
-            this->curr_ = ptr;
-
             // allocate a new block and return a ptr to the new memory
             return this->grow_(count, t);
         }
 
+        // This is the ptr to return
+        T *ptr = this->curr_;
+
+        // Advance the high-water mark
+        this->curr_ += count;
+
         return ptr;
     }