avcodec/pnm_parser: Factor out next/index compensation

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pnm_parser.c

@@ -70,19 +70,17 @@ retry:
                     c = *bs++;
             } else if (c == 'P') {
                 next = bs - pnmctx.bytestream_start + skip - 1;
-                if (pnmctx.bytestream_start != buf + skip)
-                    next -= pc->index;
                 break;
             }
         }
     } else {
         next = pnmctx.bytestream - pnmctx.bytestream_start + skip
                + av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);
-        if (pnmctx.bytestream_start != buf + skip)
+    }
+    if (next != END_NOT_FOUND && pnmctx.bytestream_start != buf + skip)
         next -= pc->index;
     if (next > buf_size)
         next = END_NOT_FOUND;
-    }
 
     if (ff_combine_frame(pc, next, &buf, &buf_size) < 0) {
         *poutbuf      = NULL;