lavc/pnm_parser: disable parsing for text based PNMs

P1, P2, and P3 are respectively the text versions of PBM, PGM and PPM
files.

We can not obtain the buffer size using av_imgage_get_buffer_size() as
every pixel in the picture will occupy a random size between 16 and 32
bits ("4 " and "231 " are such example).

Ideally, we could look for the next header (or EOF) in the bytestream,
but this commit is meant to fix a decoding regression introduced by
48ac4532d437790e56b8ed5d0d467dc88685f035.

Fix Ticket #5670

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