CDRIVER-2894 introduced three occurrences of the logic for calling isspace after checking the bounds of the argument, like:

c >= -1 && c <= 255 && isspace (c)

There are checks in libbson, once in libmongoc, and once in the common library. We should probably refactor this into one function, perhaps named isspace_safe either in bson-string.h or somewhere in common library.

Additionally, the check in common-b64.c checks with an unsigned char, making the comparison of -1 redundant and triggering an autological warning.