mp3dec: Update input buffer validation checks to handle EOS case

In case of EOS, application may call pvmp3_framedecoder() with a zero
length buffer and in such cases pvmp3_framedecoder() is expected return
NO_ENOUGH_MAIN_DATA_ERROR.
Earlier CL that added checks to validate the input buffer, was wrongly
returning SYNCH_LOST_ERROR for zero length cases. This is now addressed
by updating the error code appropriately.

Bug: 315387969
Test: atest VtsHalMediaOmxV1_0TargetAudioDecTest
(cherry picked from https://partner-android-review.googlesource.com/q/commit:3aba4cda18971c011b3ce27778e4d056688c42ad)
Merged-In: I48038143dd7c797d7a95a8e90ce56c8e659fb51d
Change-Id: I48038143dd7c797d7a95a8e90ce56c8e659fb51d