[build] Move various pointer assignments after their length checks

When performing a length check on untrusted received data, it is
preferable to assign the corresponding typed pointer only after
validating that the length is sufficient to contain the dereferenced
pointer type.  This allows the compiler to catch any unintended
dereferences before the length check has taken place, and so hardens
the code against future potential changes.

This pattern of assigning the pointer only after the corresponding
length check is already fairly widespread, but there are still large
swathes of older code that use the less safe idiom of assigning the
pointer first (generally as part of the variable declaration).

Move an assortment of pointer assignments after their corresponding
length checks, and fix the few harmless premature dereferences that
were discovered in the process (e.g. using a potentially non-existent
IPv4 source address as a debug colour stream identifier).

This is not intended to be a comprehensive update of all such pointer
assignments, merely an improvement of those sites where assignments
are easily identifiable and trivially hardened.

Signed-off-by: Michael Brown <mcb30@ipxe.org>