This is useful for our internal builds where we override the
configuration in the header files generated by autoconf.

None of these are harmful, and they are almost certainly optimized
away by the compiler. The motivation for fixing them anyway is that
we'd like to enable static analysis as part of CI, and the first step
towards that is resolving the warnings it produces at present.

Static analysis flagged this issue. Here is a minimal program which
causes a segfault within Jemalloc:
```
#include <jemalloc/jemalloc.h>

const char *malloc_conf = "prof:true";

int main() {
  mallctl("prof.prefix", NULL, NULL, NULL, 0);
}
```

Fixed by checking if `prefix` is `NULL`.

This tests the combination of the prof_recent and thread_name features.
Verified that it catches the issue being fixed in this PR.

Also explicitly set thread name in test/unit/prof_recent.  This fixes the name
testing when no default thread name is set (e.g. FreeBSD).

As pointed out in #2434, the thread_name in prof_tdata_t was changed in #2407.
This also requires an update for the prof_recent dump, specifically the emitter
expects a "char **" which is fixed in this commit.

and use it on the background thread name setting.

Static analysis flagged this. Fixed by simply checking `oldlenp`
before dereferencing it.

Static analysis flagged this. `extent_record` was passing `NULL` as the
value for `coalesced` to `extent_try_coalesce`, which in turn passes
that argument to `extent_try_coalesce_impl`, where it is written to
without checking if it is `NULL`. I can confirm from reviewing the
fleetwide coredump data that this was in fact being hit in production.