fix(column): clamp line number for legacy signs

Problem:  Legacy :sign API still allows placing signs beyond the end of
          the buffer. This is unaccounted for by the signcolumn tracking
          logic and is disallowed in general for the extmark API which
          implements it now.
Solution: Clamp legacy sign line number to the length of the buffer.

src/nvim/sign.c

@@ -126,8 +126,8 @@ static void buf_set_sign(buf_T *buf, uint32_t *id, char *group, int prio, linenr
                          | (has_hl ? MT_FLAG_DECOR_SIGNHL : 0);
 
   DecorInline decor = { .ext = true, .data.ext = { .vt = NULL, .sh_idx = decor_put_sh(sign) } };
-  extmark_set(buf, ns, id, lnum - 1, 0, -1, -1, decor, decor_flags, true,
-              false, true, true, NULL);
+  extmark_set(buf, ns, id, MIN(buf->b_ml.ml_line_count, lnum) - 1, 0, -1, -1,
+              decor, decor_flags, true, false, true, true, NULL);
 }
 
 /// For an existing, placed sign with "id", modify the sign, group or priority.