Lines 288-290 of mongo-c-driver-1.16.0/src/common/common-b64.c:

for (i = 1; i < 256; ++i) {

  ch = (unsigned char) i;
  /* Whitespaces */
  if (ch >= -1 && ch <= 255 && isspace (ch))

ch is unsigned; it's never not going to be greater than or equal to -1. Further, you're iterating in a loop from 1 to 255 in the first place; not sure where a -1 is going to sneak in there, frankly.