Overload Signature: Potential Nullptr Deref

Fix a potential nullptr in error handling.
The `it` variable is already checked in the `try`
block for non-nullptr and might therefore be `nullptr`
in this branch as well.

Found with coverity in a downstream project.

Author: ax3l

include/pybind11/pybind11.h

@@ -763,7 +763,10 @@ class cpp_function : public function {
         } else if (!result) {
             std::string msg = "Unable to convert function return value to a "
                               "Python type! The signature was\n\t";
-            msg += it->signature;
+            if (it == nullptr)
+                msg += "not found (nullptr).";
+            else
+                msg += it->signature;
             append_note_if_missing_header_is_suspected(msg);
             PyErr_SetString(PyExc_TypeError, msg.c_str());
             return nullptr;