`FREE()` checks for a `NULL` pointer, but generally httpdirfs does not
`NULL` out pointers it attempts to `FREE()` (or `free()`). As such, the
error message is misleading; make it less so in a trivial way.
Possibly a better, more invasive, change would be for `FREE()` to take a
`void** pp`, check that `*p != NULL`, `free(*p)`, and then `*p = NULL;`.
Were that done, then there would be some plausibility to the current
diagnostic message.
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