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Fix build failure reported in gcc 9.

Apparently gcc 9 is clever enough to say 'Hey, runtime field width in
an sprintf targeting a fixed-size buffer!', but not clever enough to
notice that the width was computed earlier as the max of lots of
default-width sprintfs into the same buffer (so _either_ it's safe, or
else - on a hypothetical platform with a 263-bit int - the damage was
already done).

Added a bounds check or two to keep it happy.

--- a/twiddle.c

+++ b/twiddle.c

@@ -550,6 +550,12 @@

     int i, x, y, col, maxlen;

     bool o = state->orientable;

+    /* Pedantic check: ensure buf is large enough to format an int in

+     * decimal, using the bound log10(2) < 1/3. (Obviously in practice

+     * int is not going to be larger than even 32 bits any time soon,

+     * but.) */

+    assert(sizeof(buf) >= 1 + sizeof(int) * CHAR_BIT/3);

+

     /*

      * First work out how many characters we need to display each

      * number. We're pretty flexible on grid contents here, so we

@@ -560,6 +566,11 @@

 	x = sprintf(buf, "%d", state->grid[i] / 4);

 	if (col < x) col = x;

     }

+

+    /* Reassure sprintf-checking compilers like gcc that the field

+     * width we've just computed is not now excessive */

+    if (col >= sizeof(buf))

+        col = sizeof(buf)-1;

     /*

      * Now we know the exact total size of the grid we're going to