Fix retrieval of deprecated error details

The error_stack_trace() function contains logic to exclude the Error API
internal function calls. However, when it was introduced (see commit
295526c), the code processing
E_USER_DEPRECATED errors in error_handler() was not adapted.

This causes the error handler to throw 'Notice: Undefined offset' or
to return incorrect file and line number information

Fixes #25631

core/error_api.php

@@ -231,15 +231,10 @@ function error_handler( $p_type, $p_error, $p_file, $p_line ) {
 			$t_error_type = 'DEBUG';
 			break;
 		case E_USER_DEPRECATED:
-			# Get the parent of the call that triggered the error to facilitate
-			# debugging with a more useful filename and line number
+			# Get details about the error, to facilitate debugging with a more
+			# useful message including filename and line number.
 			$t_stack = error_stack_trace();
-			if( isset( $t_stack[2] ) ) {
-				$t_caller = $t_stack[2];
-			} else {
-				# If called from main page body, there is no stack block for the function, use the page block instead
-				$t_caller = $t_stack[1];
-			}
+			$t_caller = $t_stack[0];
 
 			$t_error_type = 'WARNING';
 			$t_error_description =  error_string( $p_error )