xen/vsprintf: Avoid returning NULL from number()

xen/vsprintf: Avoid returning NULL from number()

In practice this is an unused codepath, as every caller of number() passes an
explicit base of 8, 10 or 16.  For all other uses, number() returns a pointer
between the str and end parameters, as do the other similar helper functions.

However, the fact that there is a NULL return path causes Coverity to check
whether the caller makes NULL checks on the return value, and complain.

Change the conditional return into an ASSERT().

No functional change, but this removes 21 instances of NULL_RETURN in
Coverity.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
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CC: Jan Beulich <JBeulich@suse.com>