gcc 7.0 prerelease crash

[Ironholds]

With GCC 7.0, the C++ here runs into (with ASN):

ERROR: AddressSanitizer: global-buffer-overflow on address 0x7f347bea9b80 at pc 0x7f347be84bf7 bp 0x7ffd8307b8b0 sp 0x7ffd8307b8a8
READ of size 1 at 0x7f347bea9b80 thread T0
#0 0x7f347be84bf6 in std::_Rb_tree_iterator<std::pair<char const, int> > std::_Rb_tree<char, std::pair<char const, int>, std::_Select1st<std::pair<char const, int> >, std::less, std::allocator<std::pair<char const, int> > >::M_insert<std::pair<char, int>, std::_Rb_tree<char, std::pair<char const, int>, std::_Select1st<std::pair<char const, int> >, std::less, std::allocator<std::pair<char const, int> > >::_Alloc_node>(std::Rb_tree_node_base, std::Rb_tree_node_base, std::pair<char, int>&&, std::_Rb_tree<char, std::pair<char const, int>, std::_Select1st<std::pair<char const, int> >, std::less, std::allocator<std::pair<char const, int> > >::_Alloc_node&) /usr/local/gcc7/include/c++/7.0.0/bits/stl_tree.h:1501
#1 0x7f347be84bf6 in std::pair<std::_Rb_tree_iterator<std::pair<char const, int> >, bool> std::_Rb_tree<char, std::pair<char const, int>, std::_Select1st<std::pair<char const, int> >, std::less, std::allocator<std::pair<char const, int> > >::_M_insert_unique<std::pair<char, int> >(std::pair<char, int>&&) /usr/local/gcc7/include/c++/7.0.0/bits/stl_tree.h:1864
#2 0x7f347be84bf6 in std::pair<std::_Rb_tree_iterator<std::pair<char const, int> >, bool> std::map<char, int, std::less, std::allocator<std::pair<char const, int> > >::insert<std::pair<char, int>, void>(std::pair<char, int>&&) /usr/local/gcc7/include/c++/7.0.0/bits/stl_map.h:740
#3 0x7f347be84bf6 in cgeohash::build_base32_indexes() /data/gannet/ripley/R/packages/tests-70/geohash/src/cgeohash.cpp:50
#4 0x7f347be74c92 in __static_initialization_and_destruction_0 /data/gannet/ripley/R/packages/tests-70/geohash/src/cgeohash.cpp:43
#5 0x7f347be74c92 in _GLOBAL__sub_I_cgeohash.cpp /data/gannet/ripley/R/packages/tests-70/geohash/src/cgeohash.cpp:217
#6 0x7f348a6405f9 in call_init.part.0 (/lib64/ld-linux-x86-64.so.2+0xf5f9)
#7 0x7f348a64070a in _dl_init (/lib64/ld-linux-x86-64.so.2+0xf70a)