fix(signcol): handle edge case with maximum value

50250542 failed to consider that the maximum passed to buf_signcols
is window scoped whereas the signcols value is buffer scoped. This can
lead to a bug where the signcolumn becomes incorrect if:
- global signcolumn is set to auto:N
- signcolumn in a window is changed locally to auto:M where M > N
- the buffer has a line with M or greater signs.

src/nvim/buffer_defs.h

@@ -864,6 +864,7 @@ struct file_buffer {
   sign_entry_T *b_signlist;     // list of placed signs
   int b_signcols;               // last calculated number of sign columns
   bool b_signcols_valid;        // calculated sign columns is valid
+  int b_signcols_max;           // Maximum value b_signcols is valid for.
 
   Terminal *terminal;           // Terminal instance associated with the buffer