Resolve "[CLANG18] MSAN: use-of-uninitialized-value in lib_enc/ivas_mcmasa_enc_fx.c:1325:78"

• Related issues: #2324 (closed)
• Requested reviewers: @kiene

Reason why this change is needed

• MSAN error existing. Exponent variable was not initialized.

Description of the change

• Set the exponent to zero at the same place where mantissa is set to zero.

Affected operating points

• Should be BE.

Closes #2324 (closed)