Regression introduced by 42825b68

Referring to the discussion in #459, the OSS-Fuzz test case
https://github.com/libjpeg-turbo/libjpeg-turbo/files/5597075/clusterfuzz-testcase-minimized-pngsave_buffer_fuzzer-5728375846731776.txt
created a situation in which
  cinfo->output_iMCU_row > cinfo->master->last_good_iMCU_row
but
  cinfo->input_scan_number == 1
thus causing decompress_smooth_data() to read from
prev_coef_bits_latch[], which was uninitialized.  I was unable to create
the same situation with a real JPEG image.