Resolve "Assert in RefineThresholdsUsingPitch_fx() -> GetF0()"

• Related issues: #2592 (closed)
• Requested reviewers: Markus Multrus

Reason why this change is needed

• The changes in F0 computation (ISSUE_1966) show an assert.

Description of the change

• The operation L_shl in GetF0 is changed into L_shl_sat in order to allow 0x40000000 << 1 to become 0x7FFFFFFF

Affected operating points

• BE on conditions which didn't crash before.

Closes #2592 (closed)