null pointer dereference

[QiAnXinCodeSafe]

Hi all,
This is Qianxin CodeSafe Team, we found a suspicious issue,
The stream pointer "f" may be empty at

opencv/modules/imgcodecs/perf/perf_jpeg.cpp

Line 17 in 0903061

FILE *f = fopen(filename.c_str(), "rb");

Therefore,this results in a null point dereference at

opencv/modules/imgcodecs/perf/perf_jpeg.cpp

Line 18 in 0903061

fseek(f, 0, SEEK_END);

[Kumataro]

I think this is false-positive..

opencv/modules/imgcodecs/perf/perf_jpeg.cpp

Lines 15 to 18 in d9a139f

	String filename = getDataPath("stitching/boat1.jpg");
	FILE *f = fopen(filename.c_str(), "rb");
	fseek(f, 0, SEEK_END);

getDataPath() validates argument file path to read.

opencv/modules/ts/src/ts_perf.cpp

Lines 1911 to 1919 in d9a139f

	FILE* fp = fopen(path.c_str(), "r");
	if (fp)
	fclose(fp);
	else
	{
	ADD_FAILURE() << " Requested file \"" << path << "\" does not exist.";
	throw PerfEarlyExitException();
	}
	return path;

if argument file path is unavailable(missing file or losting file permission), an exception is shown following error message and stop program In getDataPath() . So after fopen() evenif ASSERT_EQ(f, nullptr) is added, it will not be executed and be unreachable code.

[==========] Running 6 tests from 2 test cases.
[----------] Global test environment set-up.
[----------] 3 tests from JPEG
[ RUN      ] JPEG.Decode
/home/kmtr/work/opencv4/modules/ts/src/ts_perf.cpp:1916: Failure
Failed
  Requested file "/home/kmtr/work/opencv_extra4/testdata/stitching/boat1.jpg" does not exist.
termination reason:  aborted by the performance testing framework
bytesIn   =          0
bytesOut  =          0
samples   =          0 of 100
outliers  =          0
frequency =          0
[  FAILED  ] JPEG.Decode (0 ms)

[Kumataro]

However, if we are to be extremely strict, there is a non-zero possibility that the file will not be accessible during the very small time slice between calling getDataPath() and calling fopen().

Since the purpose of this program is to verify performance, in my opinion this level of strictness is not necessary.