motech-lps-30x: fix several compiler warnings

motech-lps-30x: fix several compiler warnings

Check pointers' validity before dereferencing them. Explicitly assign a
default value to variables, before conversion routines conditionally
assign the "real" value (and don't in case of conversion errors). This
avoids processing "garbage" data.

Strictly speaking I cannot see how the conversion routine returns OK and
has _not_ assigned a result. But the explicit assignment won't harm
either, and matches the fallback when the conversion fails (detectibly).
Which means that runtime behaviour won't change.

This was reported by clang's scan-build.