Fix potential nullptr dereference bugs detected by coverity

lib/Differentiator/DerivativeBuilder.cpp

@@ -392,6 +392,15 @@ namespace clad {
     }
 
     m_IndependentVar = args.back();
+    // If param is not real (i.e. floating point or integral), we cannot
+    // differentiate it.
+    // FIXME: we should support custom numeric types in the future.
+    if (!m_IndependentVar->getType()->isRealType()) {
+      diag(DiagnosticsEngine::Error, m_IndependentVar->getLocEnd(),
+           "attempted differentiation w.r.t. a parameter ('%0') which is not "
+            "of a real type", { m_IndependentVar->getNameAsString() });
+      return {};
+    }
     m_DerivativeOrder = plan.getCurrentDerivativeOrder();
     std::string s = std::to_string(m_DerivativeOrder);
     std::string derivativeBaseName;
@@ -486,17 +495,8 @@ namespace clad {
     beginBlock();
     // For each function parameter variable, store its derivative value.
     for (auto param : params) {
-      // If param is not real (i.e. floating point or integral), we cannot
-      // differentiate it.
-      // FIXME: we should support custom numeric types in the future.
-      if (!param->getType()->isRealType()) {
-        if (param != m_IndependentVar)
-          continue;
-        diag(DiagnosticsEngine::Error, PVD->getLocEnd(),
-             "attempted differentiation w.r.t. a parameter ('%0') which is not "
-             "of a real type", { m_IndependentVar->getNameAsString() });
-        return {};
-      }
+      if (!param->getType()->isRealType())
+        continue;
       // If param is independent variable, its derivative is 1, otherwise 0.
       int dValue = (param == m_IndependentVar);
       auto dParam = ConstantFolder::synthesizeLiteral(m_Context.IntTy,
@@ -843,7 +843,8 @@ namespace clad {
         auto referencedDecl = cast<VarDecl>(clonedDRE->getDecl());
         clonedDRE = cast<DeclRefExpr>(BuildDeclRef(referencedDecl));
       }
-    }
+    } else
+      clonedDRE = cast<DeclRefExpr>(Clone(DRE));
 
     if (auto VD = dyn_cast<VarDecl>(clonedDRE->getDecl())) {
       // If DRE references a variable, try to find if we know something about
@@ -1674,7 +1675,8 @@ namespace clad {
         auto referencedDecl = cast<VarDecl>(clonedDRE->getDecl());
         clonedDRE = cast<DeclRefExpr>(BuildDeclRef(referencedDecl));
       }
-    }
+    } else
+      clonedDRE = cast<DeclRefExpr>(Clone(DRE));
 
     if (auto decl = dyn_cast<VarDecl>(clonedDRE->getDecl())) {
       // Check DeclRefExpr is a reference to an independent variable.