wallet: fix buffer over-read in SQLite file magic check

If there is no terminating zero within the 16 magic bytes, the buffer would be
over-read in the std::string constructor. Fixed by using the "from buffer"
variant of the ctor (that also takes a size) rather than the "from c-string"
variant.

src/wallet/sqlite.cpp

@@ -619,8 +619,8 @@ bool IsSQLiteFile(const fs::path& path)
     file.close();
 
     // Check the magic, see https://sqlite.org/fileformat2.html
-    std::string magic_str(magic);
-    if (magic_str != std::string("SQLite format 3")) {
+    std::string magic_str(magic, 16);
+    if (magic_str != std::string("SQLite format 3", 16)) {
         return false;
     }