Explicitly initialize prevector::_union to avoid new warning

Warning from gcc 7.1 is ./prevector.h:450:25: warning:
'*((void*)(&<anonymous>)+8).prevector<28, unsigned char>::_union.prevector<28, unsigned char>::direct_or_indirect::<anonymous>.prevector<28, unsigned char>::direct_or_indirect::<unnamed struct>::indirect'
may be used uninitialized in this function [-Wmaybe-uninitialized]

src/prevector.h

@@ -220,7 +220,7 @@ public:
         }
     }
 
-    prevector() : _size(0) {}
+    prevector() : _size(0), _union{{}} {}
 
     explicit prevector(size_type n) : _size(0) {
         resize(n);