bpf: gas: avoid UB in pointer subtraction

bpf: gas: avoid UB in pointer subtraction

The PARSE_ERROR macro in md_assemble performs pointer subtraction.  If
parse_expression returns NULL then the later will be part of the
subtraction and therefore UB will be incurred.

This patch changes md_assemble to:
1. Accommodate all invocations to parse_expression to the fact it will
   return NULL when a parse error occurs.
2. Avoid UB in PARSE_ERROR.

Tested in bpf-unknown-none target / x86_64-linux-gnu host.

gas/ChangeLog:

	* config/tc-bpf.c (md_assemble): Fix to take into account that
	parse_expression can return NULL.
	(PARSE_ERROR): Avoid passing invalid length to parse_error.