Fix phpdbg segmentation fault in case of malformed input

If you were to enter "w $>" the function would crash with a segmentation
fault because last_index is still NULL at that point. Fi

Fix phpdbg segmentation fault in case of malformed input

If you were to enter "w $>" the function would crash with a segmentation
fault because last_index is still NULL at that point. Fix it by checking
for NULL and erroring out if it is.

Closes GH-10353

Signed-off-by: George Peter Banyard <girgias@php.net>

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