global: Added uchar_empty_ptr and use it instead of &uchar_nul.

This makes Coverity happier about not treating a char as an array.
For now this is a pointer to a 0, but could as well become a pointer
that crashes if dereferenced. Shouldn't be NULL anyway because clang's
-fsanitize=nonnull-attribute will complain about them.

src/auth/auth-master-connection.c

@@ -419,14 +419,14 @@ master_input_pass(struct auth_master_connection *conn, const char *args)
 			return FALSE;
 		auth_request_log_info(auth_request, "passdb", "%s", error);
 		pass_callback(PASSDB_RESULT_USER_UNKNOWN,
-			      &uchar_nul, 0, auth_request);
+			      uchar_empty_ptr, 0, auth_request);
 	} else if (conn->userdb_restricted_uid != 0) {
 		/* no permissions to do this lookup */
 		auth_request_log_error(auth_request, "passdb",
 			"Auth client doesn't have permissions to do "
 			"a PASS lookup: %s", auth_restricted_reason(conn));
 		pass_callback(PASSDB_RESULT_INTERNAL_FAILURE,
-			      &uchar_nul, 0, auth_request);
+			      uchar_empty_ptr, 0, auth_request);
 	} else {
 		auth_request_set_state(auth_request,
 				       AUTH_REQUEST_STATE_MECH_CONTINUE);

src/auth/auth-request-handler.c

@@ -829,7 +829,7 @@ void auth_request_handler_flush_failures(bool flush_all)
 		i_assert(auth_request->state == AUTH_REQUEST_STATE_FINISHED);
 		auth_request_handler_reply(auth_request,
 					   AUTH_CLIENT_RESULT_FAILURE,
-					   &uchar_nul, 0);
+					   uchar_empty_ptr, 0);
 		auth_request_unref(&auth_request);
 	}
 }

src/auth/auth-request.c

@@ -1116,7 +1116,7 @@ void auth_request_lookup_credentials_policy_continue(struct auth_request *reques
 			"passdb doesn't support credential lookups");
 		auth_request_lookup_credentials_callback(
 					PASSDB_RESULT_SCHEME_NOT_AVAILABLE,
-					&uchar_nul, 0, request);
+					uchar_empty_ptr, 0, request);
 	} else if (passdb->passdb->blocking) {
 		passdb_blocking_lookup_credentials(request);
 	} else {

src/auth/mech.c

@@ -50,7 +50,7 @@ void mech_generic_auth_initial(struct auth_request *request,
 			       const unsigned char *data, size_t data_size)
 {
 	if (data == NULL) {
-		auth_request_handler_reply_continue(request, &uchar_nul, 0);
+		auth_request_handler_reply_continue(request, uchar_empty_ptr, 0);
 	} else {
 		/* initial reply given, even if it was 0 bytes */
 		request->mech->auth_continue(request, data, data_size);

src/auth/userdb-static.c

@@ -101,7 +101,7 @@ static void static_lookup(struct auth_request *auth_request,
 		} else {
 			static_credentials_callback(
 				PASSDB_RESULT_SCHEME_NOT_AVAILABLE,
-				&uchar_nul, 0, auth_request);
+				uchar_empty_ptr, 0, auth_request);
 		}
 	} else {
 		static_lookup_real(auth_request, callback);

src/lib-imap/imap-parser.c

@@ -516,7 +516,7 @@ static bool imap_parser_read_literal_data(struct imap_parser *parser,
 	} else {
 		/* we want to save only literal size, not the literal itself. */
 		parser->literal_size_return = TRUE;
-		imap_parser_save_arg(parser, &uchar_nul, 0);
+		imap_parser_save_arg(parser, uchar_empty_ptr, 0);
 		return FALSE;
 	}
 }

src/lib-sasl/mech-login.c

@@ -51,7 +51,7 @@ mech_login_output(struct dsasl_client *_client,
 
 	switch (client->state) {
 	case STATE_INIT:
-		*output_r = &uchar_nul;
+		*output_r = uchar_empty_ptr;
 		*output_len_r = 0;
 		return 0;
 	case STATE_USER:

src/lib/istream.c

@@ -503,7 +503,7 @@ i_stream_get_data(struct istream *stream, size_t *size_r)
 
 	if (_stream->skip >= _stream->pos) {
 		*size_r = 0;
-		return &uchar_nul;
+		return uchar_empty_ptr;
 	}
 
 	if (i_stream_is_buffer_invalid(_stream)) {
@@ -528,7 +528,7 @@ i_stream_get_data(struct istream *stream, size_t *size_r)
 			_stream->skip = _stream->pos = 0;
 			stream->eof = FALSE;
 		}
-		return &uchar_nul;
+		return uchar_empty_ptr;
 	}
 
         *size_r = _stream->pos - _stream->skip;

src/lib/strfuncs.c

@@ -19,6 +19,7 @@ enum _str_trim_sides {
 };
 
 const unsigned char uchar_nul = '\0';
+const unsigned char *uchar_empty_ptr = { 0 };
 
 int i_snprintf(char *dest, size_t max_chars, const char *format, ...)
 {

src/lib/strfuncs.h

@@ -4,6 +4,7 @@
 #define MAX_INT_STRLEN ((sizeof(uintmax_t) * CHAR_BIT + 2) / 3 + 1)
 
 extern const unsigned char uchar_nul; /* (const unsigned char *)"" */
+extern const unsigned char *uchar_empty_ptr; /* non-NULL pointer that shouldn't be dereferenced. */
 
 /* Returns -1 if dest wasn't large enough, 0 if not. */
 int i_snprintf(char *dest, size_t max_chars, const char *format, ...)