iff: prevent a possible crash with broken/prepared IFF PBM

Based on fix by Peter Ross for ticket #1054.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/iff.c

@@ -553,13 +553,16 @@ static int decode_frame_ilbm(AVCodecContext *avctx,
                 memcpy(row, buf, FFMIN(avctx->width, buf_end - buf));
                 buf += avctx->width + (avctx->width % 2); // padding if odd
             }
-        } else { // IFF-PBM: HAM to PIX_FMT_BGR32
+        } else if (s->ham) { // IFF-PBM: HAM to PIX_FMT_BGR32
             for (y = 0; y < avctx->height; y++) {
                 uint8_t *row = &s->frame.data[0][ y*s->frame.linesize[0] ];
                 memcpy(s->ham_buf, buf, FFMIN(avctx->width, buf_end - buf));
                 buf += avctx->width + (avctx->width & 1); // padding if odd
                 decode_ham_plane32((uint32_t *) row, s->ham_buf, s->ham_palbuf, s->planesize);
             }
+        } else {
+            av_log_ask_for_sample(avctx, "unsupported bpp\n");
+            return AVERROR_INVALIDDATA;
         }
     }