[checked-c-convert] Minor fixes to the checked-c-convert

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#679

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This pull request fixes various issues related to checked-c-convert.

Specifically, the following issues have been fixed:

• Using calloc() will keep the pointer wild correctcomputation/checkedc-clang#8
• checked-c-convert tool: struct variable containing a checked pointer not initialized correctcomputation/checkedc-clang#3
• converter tool deletes variable arguments correctcomputation/checkedc-clang#2

Although, the number of commits looks more (because of rebasing our branch with master), the effective changes are very less.