Prevent imread from illegal memory access (Bug #2602)

The change is based on pull request #211.

modules/highgui/src/grfmt_jpeg.cpp

@@ -229,6 +229,9 @@ bool  JpegDecoder::readHeader()
             if( m_f )
                 jpeg_stdio_src( &state->cinfo, m_f );
         }
+
+        if (state->cinfo.src != 0)
+        {
             jpeg_read_header( &state->cinfo, TRUE );
 
             m_width = state->cinfo.image_width;
@@ -236,6 +239,7 @@ bool  JpegDecoder::readHeader()
             m_type = state->cinfo.num_components > 1 ? CV_8UC3 : CV_8UC1;
             result = true;
         }
+    }
 
     if( !result )
         close();