GcovData.cc: Not restoring ostream format error spotted by Coverity

Original author: rlong

CID 1506202: Not restoring ostream format

   3. format_changed: hex changes the format state of std::cerr for category basefield.
407      std::cerr << "File is not a valid *.gcno output (magic: 0x"
408                << std::hex << std::setw( 4 ) << preamble->magic
409                << ")" << std::endl;
   CID 1506202 (#1 of 1): Not restoring ostream format (STREAM_FORMAT_STATE)4. end_of_path: Changing format state of stream std::cerr for category basefield without later restoring it.