Remove 'if' from GDB_PY_HANDLE_EXCEPTION

This removes the embedded 'if' from GDB_PY_HANDLE_EXCEPTION and
GDB_PY_SET_HANDLE_EXCEPTION.  I believe this 'if' was necessary with
the old gdb try/catch macros, but it no longer is: these should only
ever be called from a 'catch' block, where it's already known that an
exception was thrown.

Simon pointed out, though, that in a few spots, these were in facts
called outside of 'catch' blocks.  This patch cleans up these spots.
I also found one spot where a redundant 'return nullptr' could be
removed.

gdb/python/py-value.c

@@ -1046,7 +1046,6 @@ get_field_type (PyObject *field)
 static PyObject *
 valpy_getitem (PyObject *self, PyObject *key)
 {
-  struct gdb_exception except;
   value_object *self_value = (value_object *) self;
   gdb::unique_xmalloc_ptr<char> field;
   struct type *base_class_type = NULL, *field_type = NULL;
@@ -1178,11 +1177,9 @@ valpy_getitem (PyObject *self, PyObject *key)
     }
   catch (gdb_exception &ex)
     {
-      except = std::move (ex);
+      GDB_PY_HANDLE_EXCEPTION (ex);
     }
 
-  GDB_PY_HANDLE_EXCEPTION (except);
-
   return result;
 }
 
@@ -1678,7 +1675,6 @@ valpy_absolute (PyObject *self)
 static int
 valpy_nonzero (PyObject *self)
 {
-  struct gdb_exception except;
   value_object *self_value = (value_object *) self;
   struct type *type;
   int nonzero = 0; /* Appease GCC warning.  */
@@ -1698,14 +1694,12 @@ valpy_nonzero (PyObject *self)
     }
   catch (gdb_exception &ex)
     {
-      except = std::move (ex);
+      /* This is not documented in the Python documentation, but if
+	 this function fails, return -1 as slot_nb_nonzero does (the
+	 default Python nonzero function).  */
+      GDB_PY_SET_HANDLE_EXCEPTION (ex);
     }
 
-  /* This is not documented in the Python documentation, but if this
-     function fails, return -1 as slot_nb_nonzero does (the default
-     Python nonzero function).  */
-  GDB_PY_SET_HANDLE_EXCEPTION (except);
-
   return nonzero;
 }