vsnprintf: Added null string unit test.

src/crt.cpp

@@ -166,7 +166,11 @@ namespace bx
 				size += writeRep(_writer, _param.fill, padding, _err);
 			}
 
-			if (_param.upper)
+			if (NULL == _str)
+			{
+				size += write(_writer, "(null)", 6, _err);
+			}
+			else if (_param.upper)
 			{
 				for (int32_t ii = 0; ii < len; ++ii)
 				{

src/string.cpp

@@ -107,8 +107,13 @@ namespace bx
 
 	size_t strnlen(const char* _str, size_t _max)
 	{
-		const char* ptr;
-		for (ptr = _str; 0 < _max && *ptr != '\0'; ++ptr, --_max) {};
+		if (NULL == _str)
+		{
+			return 0;
+		}
+
+		const char* ptr = _str;
+		for (; 0 < _max && *ptr != '\0'; ++ptr, --_max) {};
 		return ptr - _str;
 	}