[core] Handle NaN input to interpolate and step

include/mbgl/style/expression/interpolate.hpp

@@ -11,7 +11,7 @@
 
 #include <memory>
 #include <map>
-
+#include <cmath>
 
 namespace mbgl {
 namespace style {
@@ -109,9 +109,15 @@ public:
     
     EvaluationResult evaluate(const EvaluationContext& params) const override {
         const EvaluationResult evaluatedInput = input->evaluate(params);
-        if (!evaluatedInput) { return evaluatedInput.error(); }
+        if (!evaluatedInput) {
+            return evaluatedInput.error();
+        }
+
         float x = *fromExpressionValue<float>(*evaluatedInput);
-        
+        if (std::isnan(x)) {
+            return EvaluationError { "Input is not a number." };
+        }
+
         if (stops.empty()) {
             return EvaluationError { "No stops in exponential curve." };
         }