Backport ksh93v- bugfix for a possible crash in types.sh (#812)

This commit backports a bugfix from ksh93v- 2012-08-24 for a
possible crash at the first strncmp in create_type(). This crash
will occur if the size and/or alignment of Namval struct is changed
(i.e., if np->nvsize or np->nvflag is upgraded to a larger data
type).

src/cmd/ksh93/sh/nvtype.c:
- Account for NV_MINSZ when setting the 'base' string to ensure no
  invalid reads occur because of changes in the size of Namval_t.
- Additionally, store the return value from strlen in a size_t
  variable.

src/cmd/ksh93/sh/nvtype.c

@@ -481,8 +481,8 @@ static Namval_t *create_type(Namval_t *np,const char *name,int flag,Namfun_t *fp
 	n = (cp-1) -name;
 	if(dp->numnodes && dp->strsize<0)
 	{
-		char *base =  (char*)np-sizeof(Dtlink_t);
-		int m=strlen(np->nvname);
+		char *base =  (char*)np-(NV_MINSZ-sizeof(Dtlink_t));
+		size_t m=strlen(np->nvname);
 		while((nq=nv_namptr(base,++i)) && strncmp(nq->nvname,np->nvname,m)==0)
 		{
 			if(nq->nvname[m]=='.' && strncmp(name,&nq->nvname[m+1],n)==0 && nq->nvname[m+n+1]==0)