set variable 'buffer' that in base32_decode to unsigned integer type

[MuggleWei]

In base32_decode, variable 'buffer' is signed integer may lead undefined behavior when left operand.

e.g. compile and link with -fsanitize=undefined, and run test below:

const char *b32_key = "PFXWQZLMNRXXO33SNRSA";
unsigned char key[64];
base32_decode((uint8_t*)b32_key, key, sizeof(key));

get output:

runtime error: left shift of 509336089 by 5 places cannot be represented in type 'int'

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[ThomasHabets]

Thanks!