Resolve "[CLANG18] MSAN: use-of-uninitialized-value in lib_enc/ivas_mct_enc_mct_fx.c:99:27"

• Related issues: #2356 (closed)
• Requested reviewers: @tritthart

Reason why this change is needed

• Exponent nrg_e[ch] is not properly initialized for first loop iteration
• Also, the initialization of sum2_e = st->hTcxEnc->spectrum_e[0] seems not to be correct for nSubframes > 1

Description of the change

• Correctly initialize nrg_e[ch]
• Correctly initialize sum2_e

Affected operating points

• Changes are BE for all operating points

Closes #2356 (closed)