Don't rely on SHA1() returning a static buffer

Index: src/smtp/smtp_sasl_auth_cache.c
--- src/smtp/smtp_sasl_auth_cache.c.orig
+++ src/smtp/smtp_sasl_auth_cache.c
@@ -170,9 +170,10 @@ static char *smtp_sasl_auth_cache_make_key(const char 
 static char *smtp_sasl_auth_cache_make_pass(const char *password)
 {
     VSTRING *buf = vstring_alloc(2 * SHA_DIGEST_LENGTH);
+    unsigned char md[EVP_MAX_MD_SIZE];
 
     base64_encode(buf, (const char *) SHA1((const unsigned char *) password,
-					   strlen(password), 0),
+					   strlen(password), md),
 		  SHA_DIGEST_LENGTH);
     return (vstring_export(buf));
 }
