fix undefined behavior in wcsto[ld] family functions

analogous to commit b287cd74 but for
the custom FILE stream type the wcstol and wcstod family use. __toread
could be used here as well, but there's a simple direct fix to make
the buffer pointers initially valid for subtraction, so just do that
to avoid pulling in stdio exit code in programs that don't use stdio.

src/stdlib/wcstod.c

@@ -33,8 +33,7 @@ static long double wcstox(const wchar_t *s, wchar_t **p, int prec)
 	unsigned char buf[64];
 	FILE f = {0};
 	f.flags = 0;
-	f.rpos = f.rend = 0;
-	f.buf = buf + 4;
+	f.rpos = f.rend = f.buf = buf + 4;
 	f.buf_size = sizeof buf - 4;
 	f.lock = -1;
 	f.read = do_read;

src/stdlib/wcstol.c

@@ -35,8 +35,7 @@ static unsigned long long wcstox(const wchar_t *s, wchar_t **p, int base, unsign
 	unsigned char buf[64];
 	FILE f = {0};
 	f.flags = 0;
-	f.rpos = f.rend = 0;
-	f.buf = buf + 4;
+	f.rpos = f.rend = f.buf = buf + 4;
 	f.buf_size = sizeof buf - 4;
 	f.lock = -1;
 	f.read = do_read;