service update: Don't assume existing pointers in spec are valid
service update: Don't assume existing pointers in spec are valid

When updating values in the spec according to CLI flags, don't write
into the existing pointers. They may be nil. Instead, update them to
point to the new value we're writing.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>
(cherry picked from commit f9c920a1266197c2b6f0378b58f22246037fda7a)
Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

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