lac/scalapack.cc: fix an out-of-bounds write that leads to a double free

lac/scalapack.cc: fix an out-of-bounds write that leads to a double free

```
==1080297==ERROR: AddressSanitizer: attempting free on address which was not malloc()-ed: 0x60200003da70 in thread T0
    #0 0x55bcdd907b7d in operator delete(void*) (/srv/temp/testsuite-IQZ1b8kK/build/tests/scalapack/scalapack_06b.debug/scalapack_06b.debug+0x17cb7d)
    #1 0x7fc52a4d2047 in void std::__1::__libcpp_operator_delete[abi:v160006]<void*>(void*) /usr/include/c++/v1/new:276:3
    #2 0x7fc52a4d2047 in void std::__1::__do_deallocate_handle_size[abi:v160006]<>(void*, unsigned long) /usr/include/c++/v1/new:300:10
    #3 0x7fc52a4d2047 in std::__1::__libcpp_deallocate[abi:v160006](void*, unsigned long, unsigned long) /usr/include/c++/v1/new:316:14
    #4 0x7fc52a4d2047 in std::__1::allocator<int>::deallocate[abi:v160006](int*, unsigned long) /usr/include/c++/v1/__memory/allocator.h:131:13
    #5 0x7fc52a4d2047 in std::__1::allocator_traits<std::__1::allocator<int>>::deallocate[abi:v160006](std::__1::allocator<int>&, int*, unsigned long) /usr/include/c++/v1/__memory/allocator_traits.h:288:13
    #6 0x7fc52a4d2047 in std::__1::__split_buffer<int, std::__1::allocator<int>&>::~__split_buffer() /usr/include/c++/v1/__split_buffer:362:9
    #7 0x7fc52a4d2047 in std::__1::vector<int, std::__1::allocator<int>>::__append(unsigned long) /usr/include/c++/v1/vector:1049:5
    #8 0x7fc52a8282d2 in std::__1::vector<int, std::__1::allocator<int>>::resize(unsigned long) /usr/include/c++/v1/vector:1910:15
    #9 0x7fc52a8282d2 in dealii::ScaLAPACKMatrix<double>::eigenpairs_symmetric(bool, std::__1::pair<unsigned int, unsigned int> const&, std::__1::pair<double, double> const&) /srv/temp/testsuite-IQZ1b8kK/dealii/source/lac/scalapack.cc:1684:17
    #10 0x7fc52a826813 in dealii::ScaLAPACKMatrix<double>::eigenpairs_symmetric_by_index(std::__1::pair<unsigned int, unsigned int> const&, bool) /srv/temp/testsuite-IQZ1b8kK/dealii/source/lac/scalapack.cc:1446:12
    #11 0x55bcdd911907 in void test<double>(unsigned int, unsigned int, double) /srv/temp/testsuite-IQZ1b8kK/dealii/tests/scalapack/scalapack_06b.cc:134:21
    #12 0x55bcdd90ebc8 in main /srv/temp/testsuite-IQZ1b8kK/dealii/tests/scalapack/scalapack_06b.cc:207:9
    #13 0x7fc4f5250989  (/usr/lib64/libc.so.6+0x23989)
    #14 0x7fc4f5250a44 in __libc_start_main (/usr/lib64/libc.so.6+0x23a44)
    #15 0x55bcdd7f73f0 in _start (/srv/temp/testsuite-IQZ1b8kK/build/tests/scalapack/scalapack_06b.debug/scalapack_06b.debug+0x6c3f0)
```