struct/buf.c (buf_getmem*): Add an explicit `BENSURE'.

Otherwise we fail when processing a 64-bit length on a 32-bit platform:
an overlong 64-bit length gets truncated to a possibly-acceptable 32-bit
length in the implicit conversion to `size_t' in the call to `buf_get'.
Using `BENSURE' directly ensures that the comparison gets done with the
correct integer type.