glcpp: Fix undefined behaviour in glcpp

Specifically, fix this error (which is covered in existing tests):

    ../src/compiler/glsl/glcpp/pp.c:198:28: runtime error: applying non-zero offset 1 to null pointer
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/compiler/glsl/glcpp/pp.c:198:28 in

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9669>

src/compiler/glsl/glcpp/pp.c

@@ -195,11 +195,11 @@ remove_line_continuations(glcpp_parser_t *ctx, const char *shader)
 			}
 		}
 
-		search_start = backslash + 1;
-
 		if (backslash == NULL)
 			break;
 
+		search_start = backslash + 1;
+
 		/* At each line continuation, (backslash followed by a
 		 * newline), copy all preceding text to the output, then
 		 * advance the shader pointer to the character after the