bpo-9194: Fix the bounds checking in winreg.c's fixupMultiSZ() (GH-12687)

(cherry picked from commit 56ed86490c)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

PC/winreg.c

@@ -520,7 +520,7 @@ fixupMultiSZ(wchar_t **str, wchar_t *data, int len)
     Q = data + len;
     for (P = data, i = 0; P < Q && *P != '\0'; P++, i++) {
         str[i] = P;
-        for(; *P != '\0'; P++)
+        for (; P < Q && *P != '\0'; P++)
             ;
     }
 }