getdigits: introduce strict, def parameters

Problem:
During a refactor long ago, we changed the `getdigits_*` familiy of
functions to abort on overflow.  But this is often wrong, because many
of these codepaths are handling user input.

Solution:
Decide at each call-site whether to use "strict" mode.

fix #5555

src/nvim/ops.c

@@ -5714,7 +5714,7 @@ bool prepare_yankreg_from_object(yankreg_T *reg, String regtype, size_t lines)
       return false;
     }
     const char *p = regtype.data+1;
-    reg->y_width = getdigits_int((char_u **)&p)-1;
+    reg->y_width = getdigits_int((char_u **)&p, false, 1) - 1;
     if (regtype.size > (size_t)(p-regtype.data)) {
       return false;
     }