tdb: fix warnings with gcc -O3, and one test crash.

tdb: fix warnings with gcc -O3, and one test crash.

unsigned char * is allowed to alias, so we use that for byte reversing
rather than uint32_t.  This is portable.

Remove warnings about ignoring pread/pwrite return values.

And initialize tdb before setjmp: with optimization, gcc validly saw this
as NULL and crashed.