dos: buffer-overrun and resource-leak fixes

- fgets: bail early on zero-length buffer, decrement once
- getvar: close file handle on the size-lookup failure path
- runhandler: UnLock dir on the LoadSeg success path
- splitname: bail early on zero-size buffer

Author: RMortenB

rom/dos/fgets.c

@@ -54,7 +54,10 @@
     ULONG len = 0;
     LONG  c;
 
-    buflen--;
+	if(!buflen)
+		return NULL;
+
+	buflen--;
 
     do
     {

rom/dos/getvar.c

@@ -237,6 +237,7 @@ static LONG getvar_from(const char *name, const char *volume, STRPTR buffer, LON
         {
             D(bug("GetVar: can't find size\n"));
 
+	        Close(file);
             return -1;
         }
 

rom/dos/runhandler.c

@@ -73,6 +73,7 @@ struct MsgPort *RunHandler(struct DeviceNode *deviceNode, const char *path, stru
                     D(bug("[RunHandler] LoadSeg(\"L:%s\")\n", cp));
                     deviceNode->dn_SegList = LoadSeg(cp);
                     CurrentDir(olddir);
+					UnLock(dir);
                 }
             }
         }

rom/dos/splitname.c

@@ -69,6 +69,9 @@
 {
     AROS_LIBFUNC_INIT
 
+	if(!size)
+		return -1;
+
     size --;
 
     name += oldpos;