Fix last change in image.c

Fix last change in image.c

* src/image.c (svg_load_image): Don't call Lisp to remove trailing
whitespace from librsvg error messages, do it in C instead; this
reduces consing and is much cleaner.  Don't display empty error
reason if librsvg happens to return an empty message text.
(syms_of_image): Don't DEFSYM string-trim-right.  (Bug#57755)