lib-mail: message-parser: Fixed Clang 6.0 compiler warning.

Warning was:

index-mail.c:1182:3: warning: arithmetic on a null pointer treated as a cast
from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
                message_parser_parse_body(data->parser_ctx,
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../src/lib-mail/message-parser.h:95:28: note: expanded from macro
'message_parser_parse_body'
                (void *)((char *)context + CALLBACK_TYPECHECK(callback, \
                         ~~~~~~~~~~~~~~~ ^

src/lib-mail/message-parser.h

@@ -92,7 +92,7 @@ void message_parser_parse_body(struct message_parser_ctx *ctx,
 #define message_parser_parse_body(ctx, callback, context) \
 	  message_parser_parse_body(ctx, \
 		(message_part_header_callback_t *)callback, \
-		(void *)((char *)context + CALLBACK_TYPECHECK(callback, \
+		(void *)((uintptr_t)context + CALLBACK_TYPECHECK(callback, \
 			void (*)(struct message_part *, \
 				struct message_header_line *, typeof(context)))))