build: resolve FTBFS when GLIBCXX_DEBUG is set

exch/ews/structures.cpp: In member function 'const TAGGED_PROPVAL*
gromox::EWS::Structures::sShape::writes(uint32_t) const':
exch/ews/structures.cpp:897:34: error: operands to '?:' have different
types '__gnu_cxx::__normal_iterator<const TAGGED_PROPVAL*,
std::__cxx1998::vector<TAGGED_PROPVAL,
std::allocator<TAGGED_PROPVAL>>>' and 'std::nullptr_t'

Author: jengelh

.github/workflows/build.yml

@@ -35,11 +35,11 @@ jobs:
     - name: buildall
       run: |
         set -e
-        pushd vmime && cmake . -DVMIME_SENDMAIL_PATH:STRING="/usr/sbin/sendmail" -DVMIME_BUILD_SAMPLES:BOOL=OFF -DVMIME_HAVE_TLS_SUPPORT:BOOL=ON -DVMIME_BUILD_STATIC_LIBRARY:BOOL=OFF && make "-j$(nproc)" && sudo make install && popd
-        pushd libHX && ./qconf && make "-j$(nproc)" && sudo make install && popd
+        cd vmime && cmake . -DVMIME_SENDMAIL_PATH:STRING="/usr/sbin/sendmail" -DVMIME_BUILD_SAMPLES:BOOL=OFF -DVMIME_HAVE_TLS_SUPPORT:BOOL=ON -DVMIME_BUILD_STATIC_LIBRARY:BOOL=OFF && make "-j$(nproc)" && sudo make install && cd -
+        cd libHX && ./qconf && make "-j$(nproc)" && sudo make install && cd -
         ./qconf
         LD_LIBRARY_PATH=/usr/local/lib make "-j$(nproc)"
         LD_LIBRARY_PATH=/usr/local/lib make install DESTDIR="$PWD/rt" && rm -Rf rt
         make clean
-        LD_LIBRARY_PATH=/usr/local/lib make "-j$(nproc)" CPPFLAGS=-DCOMPILE_DIAG
+        LD_LIBRARY_PATH=/usr/local/lib make "-j$(nproc)" CPPFLAGS="-DCOMPILE_DIAG -D_GLIBCXX_DEBUG"
         make distclean

exch/ews/structures.cpp

@@ -921,7 +921,7 @@ TPROPVAL_ARRAY sShape::write() const
 const TAGGED_PROPVAL* sShape::writes(uint32_t tag) const
 {
 	auto it = std::find_if(wProps.begin(), wProps.end(), [=](const TAGGED_PROPVAL& tp){return tp.proptag == tag;});
-	return it != wProps.end() ? it.base() : nullptr;
+	return it != wProps.end() ? &*it : nullptr;
 }
 
 /**