Fix nested error handling in PG_FINALLY

We need to pop the error stack before running the user-supplied
PG_FINALLY code.  Otherwise an error in the cleanup code would end up
at the same sigsetjmp() invocation and result in an infinite error
handling loop.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/95a822c3-728b-af0e-d7e5-71890507ae0c%402ndquadrant.com

src/backend/utils/adt/xml.c

@@ -3868,7 +3868,7 @@ xml_xmlnodetoxmltype(xmlNodePtr cur, PgXmlErrorContext *xmlerrcxt)
 
 			result = xmlBuffer_to_xmltype(buf);
 		}
-		PG_FINALLY()
+		PG_FINALLY();
 		{
 			if (nodefree)
 				nodefree(cur_copy);

src/include/utils/elog.h

@@ -338,14 +338,16 @@ extern PGDLLIMPORT ErrorContextCallback *error_context_stack;
 		} \
 		else \
 			_do_rethrow = true; \
-		{
+		{ \
+			PG_exception_stack = _save_exception_stack; \
+			error_context_stack = _save_context_stack
 
 #define PG_END_TRY()  \
 		} \
-		PG_exception_stack = _save_exception_stack; \
-		error_context_stack = _save_context_stack; \
 		if (_do_rethrow) \
 				PG_RE_THROW(); \
+		PG_exception_stack = _save_exception_stack; \
+		error_context_stack = _save_context_stack; \
 	} while (0)
 
 /*