Avoid some new gcc warnings in 15

src/core_read.cpp

@@ -27,7 +27,7 @@ CScript ParseScript(const std::string& s)
 
     if (mapOpNames.empty())
     {
-        for (int op = 0; op <= MAX_OPCODE; op++)
+        for (unsigned int op = 0; op <= MAX_OPCODE; op++)
         {
             // Allow OP_RESERVED to get into mapOpNames
             if (op < OP_NOP && op != OP_RESERVED)

src/prevector.h

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