Fix line search to avoid non-finite gradients

Related to stan-dev#3306

Modify the `WolfeLineSearch` function in `src/stan/optimization/bfgs_linesearch.hpp` to handle non-finite gradients.

* Check if the function value `func_val` is finite.
* Check if the gradient `gradx1` is finite.
* If either the function value or the gradient is non-finite, restart the line search with a smaller step size.

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src/stan/optimization/bfgs_linesearch.hpp

@@ -253,7 +253,7 @@ int WolfeLineSearch(FunctorType &func, Scalar &alpha, XType &x1,
 
     x1.noalias() = x0 + alpha1 * p;
     ret = func(x1, func_val, gradx1);
-    if (ret != 0) {
+    if (ret != 0 || !std::isfinite(func_val) || !gradx1.allFinite()) {
       if (lsRestarts >= maxLSRestarts) {
         retCode = 1;
         break;