avcodec/pnm: Avoid structure pointer dereferences in inner loop in pnm_get()

Improves speed from 5.4 to 4.2 seconds
Fixes: 13149/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGM_fuzzer-5760833622114304
Fixes: 13166/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-5763216322330624

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pnm.c

@@ -36,13 +36,15 @@ static void pnm_get(PNMContext *sc, char *str, int buf_size)
 {
     char *s;
     int c;
+    uint8_t *bs  = sc->bytestream;
+    const uint8_t *end = sc->bytestream_end;
 
     /* skip spaces and comments */
-    while (sc->bytestream < sc->bytestream_end) {
-        c = *sc->bytestream++;
+    while (bs < end) {
+        c = *bs++;
         if (c == '#')  {
-            while (c != '\n' && sc->bytestream < sc->bytestream_end) {
-                c = *sc->bytestream++;
+            while (c != '\n' && bs < end) {
+                c = *bs++;
             }
         } else if (!pnm_space(c)) {
             break;
@@ -50,12 +52,13 @@ static void pnm_get(PNMContext *sc, char *str, int buf_size)
     }
 
     s = str;
-    while (sc->bytestream < sc->bytestream_end && !pnm_space(c)) {
+    while (bs < end && !pnm_space(c)) {
         if ((s - str)  < buf_size - 1)
             *s++ = c;
-        c = *sc->bytestream++;
+        c = *bs++;
     }
     *s = '\0';
+    sc->bytestream = bs;
 }
 
 int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)