Fixing real precision by limiting precision to 15 digits. (osquery#8355)

Fixes osquery#8301 

The previous fix was incomplete -- there were still some numbers that were being improperly converted into string.

This fix limits double-to-string conversion to significant digits only, without an extra non-significant digit.

osquery/utils/conversions/castvariant.h

@@ -28,7 +28,13 @@ class CastVisitor : public boost::static_visitor<std::string> {
 
   std::string operator()(const double& d) const {
     std::ostringstream ss;
-    ss << std::setprecision(std::numeric_limits<double>::digits10 + 1) << d;
+    // SQLite supports 15 significant digits.
+    // The value of std::numeric_limits<T>::digits10 is the number of base-10
+    // digits that can be represented by the type T without change, that is, any
+    // number with this many significant decimal digits can be converted to a
+    // value of type T and back to decimal form, without change due to rounding
+    // or overflow. (from cppreference.com)
+    ss << std::setprecision(std::numeric_limits<double>::digits10) << d;
     std::string s = ss.str();
     if (s.find('.') == std::string::npos) {
       s += ".0";