Fbackward_prefix_chars: stay within buffer bounds
The commit 1fd3172d "(Fbackward_prefix_chars): Set point properly while scanning" (1998-03-18), moved the check against of the position against the buffer beginning out the loop condition so that we might end up checking the syntax of characters before the beginning of the buffer. This can cause segfaults or trigger a "Point before start of properties" error in `update_interval' (called indirectly from `char_quoted'). * src/syntax.c (Fbackward_prefix_chars): Stop the loop when beginning of buffer is reached (Bug #3552, Bug #17132, Bug #19379).
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