basic_parser: make sentinel() return a unique pointer

Right now, sentinel() casts the `basic_parser` pointer (`this`) to
`const char *`, but that pointer is not unique if the input buffer
happens to be placed right before the `basic_parser_impl` instance -
the end of that buffer then has the same address as `basic_parser`.

Example code:

```
  struct {
    char buffer[8]{"{\"12345\""};
    boost::json::stream_parser p;
  } s;
  s.p.write(s.buffer, sizeof(s.buffer));
  s.p.write(":0}", 3);
```

This stops parsing at the end of the buffer, and then the
`incomplete()` check in `parse_string()` will return true; the second
`write()` call will crash with assertion failure:

> boost/json/basic_parser_impl.hpp:1016: const char* boost::json::basic_parser<Handler>::parse_unescaped(const char*, std::integral_constant<bool, StackEmpty_>, std::integral_constant<bool, AllowComments_>, bool) [with bool StackEmpty_ = true; bool IsKey_ = true; Handler = boost::json::detail::handler]: Assertion `*cs == '\x22'' failed.

This changes `sentinel()` by adding 1 to guaranteed that the sentinel
pointer is unique even if the input buffers borders on this object.

include/boost/json/basic_parser_impl.hpp

@@ -217,8 +217,10 @@ const char*
 basic_parser<Handler>::
 sentinel()
 {
+    // the "+1" ensures that the returned pointer is unique even if
+    // the given input buffer borders on this object
     return reinterpret_cast<
-        const char*>(this);
+        const char*>(this) + 1;
 }
 
 template<class Handler>

test/stream_parser.cpp

@@ -17,6 +17,7 @@
 
 #include <sstream>
 #include <iostream>
+#include <cstring>
 
 #include "parse-vectors.hpp"
 #include "test.hpp"
@@ -1253,6 +1254,19 @@ R"xx({
         BOOST_TEST(serialize(t.jv) == "[]");
     }
 
+    // https://github.com/boostorg/json/pull/814
+    void
+    testSentinelOverlap()
+    {
+        struct {
+            char buffer[8];
+            boost::json::stream_parser p;
+        } s;
+        memcpy(s.buffer, "{\"12345\"", 8);
+        s.p.write(s.buffer, sizeof(s.buffer));
+        s.p.write(":0}", 3);
+    }
+
     //------------------------------------------------------
 
     void
@@ -1276,6 +1290,7 @@ R"xx({
         testDupeKeys();
         testIssue15();
         testIssue45();
+        testSentinelOverlap();
     }
 };