Fix CRYPTO_strdup

The function CRYPTO_strdup (aka OPENSSL_strdup) fails to check the return
value from CRYPTO_malloc to see if it is NULL before attempting to use it.
This patch adds a NULL check.

RT3786
Signed-off-by: NMatt Caswell <matt@openssl.org>
(cherry picked from commit 37b0cf936744d9edb99b5dd82cae78a7eac6ad60)
Reviewed-by: NRich Salz <rsalz@openssl.org>
(cherry picked from commit 20d21389c8b6f5b754573ffb6a4dc4f3986f2ca4)

crypto/mem.c

@@ -364,6 +364,9 @@ char *CRYPTO_strdup(const char *str, const char *file, int line)
 {
     char *ret = CRYPTO_malloc(strlen(str) + 1, file, line);
 
+    if (ret == NULL)
+        return NULL;
+
     strcpy(ret, str);
     return ret;
 }