[llvm-readobj] - Fix the invalid dumping of the dynamic sections without terminating DT_NULL entry.

This is https://bugs.llvm.org/show_bug.cgi?id=40861,

Previously llvm-readobj would print the DT_NULL sometimes
for the dynamic section that has no terminator entry.

The logic of printDynamicTable was a bit overcomplicated.
I rewrote it slightly to fix the issue and commented.

Differential revision: https://reviews.llvm.org/D58716

llvm-svn: 355073