remove an useless null check in S_copy_little_tm_to_big_TM

All callers of S_copy_little_tm_to_big_TM pass a true variable src.
Checking for null is pointless. On Win32 32 bit VC 2003 -O1 -GL,
perl517.dll's .text section went from 0xCO13F to 0xC012F bytes long. It can
be argued that the compiler should have figured this out on its own, but VC
for whatever reason didn't. Also pretty the indenting and align the
assignments. The null check blames to commit 806a119aef .
Added asserts() to check the arguments to S_copy_little_tm_to_big_TM.

The original version just zeroed dest if src == NULL, but that code path
was never used.  (gcc -Os inlined the function and optimized the test
away anyway.)