Fix FES stack frame access for internal errors

Author: davepagurek

src/core/friendly_errors/fes_core.js

@@ -593,7 +593,7 @@ function fesCore(p5, fn){
 
         // get the function just above the topmost frame in the friendlyStack.
         // i.e the name of the library function called from user's code
-        const func = stacktrace[friendlyStack[0].frameIndex - 2].functionName
+        const func = stacktrace[friendlyStack[0].frameIndex - 1].functionName
           .split('.')
           .slice(-1)[0];