Create some convenience functions for validating lib/dir/grp/sub nums

There are lot of places in the code where subnums and dirnums (especially) are compared against total_subs and/or total_dirs or >= 0 without a ton of consistency. We should migrate to use these functions for validity-checking going forward.

src/sbbs3/scfglib.h

@@ -66,6 +66,11 @@ int		getlibnum(scfg_t*, const char* code);
 int		getsubnum(scfg_t*, const char* code);
 int		getgrpnum(scfg_t*, const char* code);
 
+BOOL	is_valid_dirnum(scfg_t*, int);
+BOOL	is_valid_libnum(scfg_t*, int);
+BOOL	is_valid_subnum(scfg_t*, int);
+BOOL	is_valid_grpnum(scfg_t*, int);
+
 faddr_t* nearest_sysfaddr(scfg_t*, faddr_t*);
 
 #ifdef __cplusplus

src/sbbs3/scfglib1.c

@@ -826,6 +826,26 @@ int getgrpnum(scfg_t* cfg, const char* code)
 	return i;
 }
 
+BOOL is_valid_dirnum(scfg_t* cfg, int dirnum)
+{
+	return (dirnum >= 0) && (cfg != NULL) && (dirnum < cfg->total_dirs);
+}
+
+BOOL is_valid_libnum(scfg_t* cfg, int libnum)
+{
+	return (libnum >= 0) && (cfg != NULL) && (libnum < cfg->total_libs);
+}
+
+BOOL is_valid_subnum(scfg_t* cfg, int subnum)
+{
+	return (subnum >= 0) && (cfg != NULL) && (subnum < cfg->total_subs);
+}
+
+BOOL is_valid_grpnum(scfg_t* cfg, int grpnum)
+{
+	return (grpnum >= 0) && (cfg != NULL) && (grpnum < cfg->total_grps);
+}
+
 faddr_t* nearest_sysfaddr(scfg_t* cfg, faddr_t* addr)
 {
 	uint i;