Shifting (signed) integer left could trigger undefined behaviour when
the shifted value would overflow into the sign bit (e.g. 2048).
The issue was found when using AFL++ and UBSAN:
message.c:2274:33: runtime error: left shift of 2048 by 20 places cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior message.c:2274:33 in
a347641782