check for negative value in ptr_lstrip() (#988)

* check for negative value in ptr_lstrip()

This fixes parsing of DEHACKED in D2ISOv2.wad with debug version of CRT. If the
value < 0 the behavior of `isspace()` is undefined according to the
documentation.

* disable vcpkg update

src/d_deh.c

@@ -3137,7 +3137,7 @@ void rstrip(char *s)  // strip trailing whitespace
 //
 char *ptr_lstrip(char *p)  // point past leading whitespace
 {
-  while (isspace(*p))
+  while (*p >= 0 && isspace(*p))
     p++;
   return p;
 }