parser: address Code Analysis warning

Nice catch: when calling with basic_bol::match(nullptr, 5, 5), the
method addresses null[4].

Turned this condition into debug assert, as (nullptr, 5, 5) is invalid
input parameter combination.

include/stdex/parser.hpp

@@ -625,6 +625,7 @@ namespace stdex
 				_In_ size_t end = SIZE_MAX,
 				_In_ int flags = match_default)
 			{
+				_Assume_(text || !end);
 				_Assume_(text || start >= end);
 				bool r = start == 0 || (start <= end && stdex::islbreak(text[start - 1]));
 				if ((r && !m_invert) || (!r && m_invert)) {