Move zero-termination out of as_utf8_end

as_utf8_end was used with std::string, where writing an extra zero-terminating
character should *probably* always work (at least if size is positive) but is
not ideal.

The only place that needed to zero-terminate was convert_path_heap.

src/pugixml.cpp

@@ -1691,9 +1691,6 @@ PUGI__NS_BEGIN
 	
 		assert(begin + size == end);
 		(void)!end;
-
-		// zero-terminate
-		buffer[size] = 0;
 	}
 	
 #ifndef PUGIXML_NO_STL
@@ -4295,6 +4292,9 @@ PUGI__NS_BEGIN
 		// second pass: convert to utf8
 		as_utf8_end(result, size, str, length);
 
+		// zero-terminate
+		result[size] = 0;
+
 		return result;
 	}