avcodec/targa: Check colors vs. available space

Fixes: Timeout (37sec -> 52ms)
Fixes: 18892/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TARGA_fuzzer-5739537854889984

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

libavcodec/targa.c

@@ -132,12 +132,6 @@ static int decode_frame(AVCodecContext *avctx,
     h         = bytestream2_get_le16(&s->gb);
     bpp       = bytestream2_get_byte(&s->gb);
 
-    if (bytestream2_get_bytes_left(&s->gb) <= idlen) {
-        av_log(avctx, AV_LOG_ERROR,
-                "Not enough data to read header\n");
-        return AVERROR_INVALIDDATA;
-    }
-
     flags     = bytestream2_get_byte(&s->gb);
 
     if (!pal && (first_clr || colors || csize)) {
@@ -146,6 +140,12 @@ static int decode_frame(AVCodecContext *avctx,
         first_clr = colors = csize = 0;
     }
 
+    if (bytestream2_get_bytes_left(&s->gb) < idlen + 2*colors) {
+        av_log(avctx, AV_LOG_ERROR,
+                "Not enough data to read header\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     // skip identifier if any
     bytestream2_skip(&s->gb, idlen);