expr: fix buffer overflows in data value setters

expr: fix buffer overflows in data value setters

The data value setters memcpy() to a fixed-size buffer, but its very easy
to make nft pass too-larger values.  Example:
  @th,160,1272 gt 0

ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60b000[..]

Truncate the copy instead of corrupting the heap.
This needs additional fixes on nft side to reject such statements with a
proper error message.

Signed-off-by: Florian Westphal <fw@strlen.de>