Imgcodecs: Fixed a bug with checkSignature in GdalDecoder

In cases where the signaure string contains a terminating character,
the std::string member function size returns a smaller value than the
allocated string. In these cases, if you then try to use substr,
you will get an out_of_range exception. This patch remedies the problem.

modules/imgcodecs/src/grfmt_gdal.cpp

@@ -559,7 +559,7 @@ bool GdalDecoder::checkSignature( const String& signature )const{
 
 
     // look for NITF
-    std::string str = signature.c_str();
+    std::string str(signature);
     if( str.substr(0,4).find("NITF") != std::string::npos ){
         return true;
     }