readelf: Make sure print_form_data always consumes DW_FORM_strx[1234] data.

readelf: Make sure print_form_data always consumes DW_FORM_strx[1234] data.

Found by afl-fuzz. When printing DW_FORM_strx[1234] data eu-readelf didn't
increase readp which meant eu-readelf would keep printing the same line
dirs or files encoded with strx[1234] names. This meant that for insane
large dir or file counts eu-readelf would just keep printing endlessly
because we never reached and of the .debug_line buffer.

Signed-off-by: Mark Wielaard <mark@klomp.org>