Silence uninitialized value false positives

Silence uninitialized value false positives

In base32_decode_char the GCC 12 static analyser fails to determine
that ctx->val[1], ctx->val[3], ctx->val[4] and ctx->val[6] are
assigned values by the previous call to base32_decode_char.  Initialise
ctx->val to zeros when initalising the rest of ctx to silence the
false positive.