avcodec/pnm: Optimize reading loop in pnm_get()

Fixes: Timeout 13149 (5sec -> 3sec), 13166 (11sec -> 7sec), 13430 (5sec -> 3sec)
Fixes: 13149/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGM_fuzzer-5760833622114304
Fixes: 13166/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-5763216322330624
Fixes: 13430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PPM_fuzzer-5758658334425088

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

libavcodec/pnm.c

@@ -52,12 +52,13 @@ static void pnm_get(PNMContext *sc, char *str, int buf_size)
     }
 
     s = str;
-    while (bs < end && !pnm_space(c)) {
-        if ((s - str)  < buf_size - 1)
-            *s++ = c;
+    while (bs < end && !pnm_space(c) && (s - str) < buf_size - 1) {
+        *s++ = c;
         c = *bs++;
     }
     *s = '\0';
+    while (bs < end && !pnm_space(c))
+        c = *bs++;
     sc->bytestream = bs;
 }