box: Avoid using Statements and Declarations in Expressions.

This feature is an extension of some compilers and not included in C99.

Also cosmetics in isom_update_box_size().
sample: Avoid referencing uninitialized pointers.
fragment: Initialize TSAP, TDEC, TEPT and TPT always.

This initialization is redundant since these are unused when uninitialized
and are initialized always in used cases, but unclever compilers may
complain that these variables may be uninitialized.