Use simpler sizeof expressions

[kojiws]

Description

The sizeof(*ptr) expressions are a bit confusing when ptr == NULL. So this PR replaces them with sizeof(TYPE) expression instead to make the meaning more clear.

Fixes zd#

Testing

./configure --enable-smallstack && make test

Checklist

• [ ] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation