ncurses 6.1 _nc_printf_string_sp NULL screen pointer handling

[Steven Noonan]

It seems that the majority of the functions in ncurses 6.1 are able to handle a
NULL screen pointer by simply returning an error code.

This call stack does not handle it properly, however:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b9045f in _nc_printf_string_sp (sp=0x0, fmt=0x8865a0 "%s", 
ap=0x7fffffffa070) at ../ncurses/./base/safe_sprintf.c:251
251             if (screen_lines(SP_PARM) > MyRows || screen_columns(SP_PARM) > 
MyCols) {
(gdb) bt full
#0  0x00007ffff7b9045f in _nc_printf_string_sp (sp=0x0, fmt=0x8865a0 "%s", 
ap=0x7fffffffa070) at ../ncurses/./base/safe_sprintf.c:251
       result = 0x0
#1  0x00007ffff7b86b6a in vwprintw (win=0x0, fmt=0x8865a0 "%s", 
argp=0x7fffffffa070) at ../ncurses/./base/lib_printw.c:143
       buf = 0x0
       code = -1
       sp = 0x0
#2  0x00007ffff7b868e6 in wprintw (win=0x0, fmt=0x8865a0 "%s") at 
../ncurses/./base/lib_printw.c:80
       argp = {{
           gp_offset = 16,  
           fp_offset = 48,  
           overflow_arg_area = 0x7fffffffa150,  
           reg_save_area = 0x7fffffffa090
         }}
       code = 0

It seems like this would fix the issue (works locally at any rate):

diff --git a/ncurses/base/safe_sprintf.c b/ncurses/base/safe_sprintf.c
index 34abd2f..d0dbefd 100644
--- a/ncurses/base/safe_sprintf.c
+++ b/ncurses/base/safe_sprintf.c
@@ -247,6 +247,8 @@ NCURSES_SP_NAME(_nc_printf_string) (NCURSES_SP_DCLx
 #else
 #define MyCols _nc_globals.safeprint_cols
 #define MyRows _nc_globals.safeprint_rows
+       if (!SP_PARM)
+               return NULL;
 
        if (screen_lines(SP_PARM) > MyRows || screen_columns(SP_PARM) > MyCols) 
{
            if (screen_lines(SP_PARM) > MyRows)