tui-disasm: Fix line buffer size calculation

tui-disasm: Fix line buffer size calculation

The code that fills the TUI disassembly window content first calculates
the maximum full length of a displayed disassembly line.  This
calculation typically yields the wrong result.  The result is too large,
so the bug does not cause any run-time failures, but unnecessary
confusion for the reader.  This patch fixes the calculation.

gdb/ChangeLog:

	* tui/tui-disasm.c (tui_set_disassem_content): Fix calculation of
	the longest disassembly line's length.