Backport ksh93v- bugfix for the crash in types.sh

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). Thus far I
can't reproduce this bug on the dev branch without changing the size
of the Namval_t struct, but the root cause of the bug should be fixed
nonetheless for correctness.

I encountered this bug while experimenting with changing the size of
Namval_t, which at the moment is primarily relevant to the unfinished
local builtin (which needs to store a new NV_DYNAMIC flag in the
variable node's nvflag set). Non-upstreamed commits for reference:
3ca5470 (local-builtin branch)
798d463 (expand-nvflags branch)
e390adf (earlier iteration of
                                                 this commit)

Crash trace from ASan (unlike the one in the earlier expand-nvflags
commit, this was retested against 0510264 with np->nvsize
changed to uint64_t; it can also be reproduced if np->nvflags
is enlargened instead):
=================================================================
==186087==ERROR: AddressSanitizer: SEGV on unknown address 0x000000001340 (pc 0x79c3ece88110 bp 0x7ffe576bd670 sp 0x7ffe576bcde0 T0)
==186087==The signal is caused by a READ memory access.
    #0 0x79c3ece88110 in strncmp /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:491
    ksh93#1 0x79c3ec3460d0 in create_type /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/nvtype.c:486
    ksh93#2 0x79c3ec4e2301 in create_tree /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/nvtree.c:74
    ksh93#3 0x79c3ec4bb610 in nv_create /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/name.c:1238
    ksh93#4 0x79c3ec4bf531 in nv_open /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/name.c:1451
    ksh93#5 0x79c3ec4b0202 in nv_setlist /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/name.c:608
    ksh93#6 0x79c3ec5819db in sh_exec /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:1076
    ksh93#7 0x79c3ec2ca5ee in b_dot_cmd /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/misc.c:324

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)