Fix minor issues with functions that dump glyph matrices.

 src/xdisp.c (Fdump_glyph_matrix, Fdump_frame_glyph_matrix): Doc fix.
 (Fdump_frame_glyph_matrix): Don't segfault if invoked on a GUI
 frame; instead, print an error message.
src/window.c (window_scroll_pixel_based): Fix a typo in a comment.