Resolve "FIX_1585_ASAN_FORMAT_SW_ALT / Format switching memory changes cause MSAN/ASAN errors in decoder usage"

• Related issues: #1593
• Requested reviewers: @vaclav

Reason why this change is needed

• Prevents attempting to free a garbage pointer if the decoder is opened and immediately closed

Description of the change

• Set the pointer to NULL at init

Affected operating points

• Decoder API

Related to #1593