avcodec/targa: Skip hflip on blank images

Fixes: timeout with 810/clusterfuzz-testcase-5249282825256960

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

libavcodec/targa.c

@@ -265,7 +265,6 @@ static int decode_frame(AVCodecContext *avctx,
                 line = advance_line(dst, line, stride, &y, h, interleave);
             } while (line);
         }
-    }
 
         if (flags & TGA_RIGHTTOLEFT) { // right-to-left, needs horizontal flip
             int x;
@@ -290,6 +289,8 @@ static int decode_frame(AVCodecContext *avctx,
                 }
             }
         }
+    }
+
 
     *got_frame = 1;