Undefined behavior in base64.h due to storing EOF in char type

[hcho3]

Related: dmlc/xgboost#3976.

Currently, base64 stream stores EOF in a char variable, which may cause undefined behavior for ARM targets.

Details. std::istream.get() and getchar() have return type int, not char, because both may return EOF, which is commonly -1. Storing EOF in a char results in undefined behavior. The char type is usually signed on x86 systems but unsigned on ARM systems (see http://blog.cdleary.com/2012/11/arm-chars-are-unsigned-by-default/), so the undefined behavior will only be apparent when compiled on ARM.

Aside. AWS has recently begun offering ARM-backed EC2 instances.

[hcho3]

#2255 should address this issue.