fix FunctionBuiltins: __truffle_source__ (methods allow arbitrary callables as functions)

Author: cosminbasca

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/function/FunctionBuiltins.java

@@ -249,10 +249,13 @@ Object doFunction(PFunction function) {
         }
 
         @Specialization
-        Object doMethod(PMethod function) {
-            String sourceCode = function.getFunction().getSourceCode();
-            if (sourceCode != null) {
-                return sourceCode;
+        Object doMethod(PMethod method) {
+            Object function = method.getFunction();
+            if (function instanceof PFunction) {
+                String sourceCode = ((PFunction) function).getSourceCode();
+                if (sourceCode != null) {
+                    return sourceCode;
+                }
             }
             return PNone.NONE;
         }