diff-delta: set size out-parameter to 0 for NULL delta

diff-delta: set size out-parameter to 0 for NULL delta

When we cannot generate a delta, we return NULL but leave delta_size
untouched. This is generally OK, as callers rely on NULL to decide if
the output is usable or not. But it can confuse compilers; in
particular, gcc 9.2.1 with "-flto -O3" complains in fast-import's
store_object() that delta_len may be used uninitialized.

Let's change the diff-delta code to set the size explicitly to 0 for a
NULL return. That silences the compiler and makes it easier to reason
about the result.

Reported-by: Stephan Beyer <s-beyer@gmx.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>