openssl: fix use of uninitialized buffer

Make sure that the error buffer is always initialized and simplify the
use of it to make the logic easier.

Bug: https://github.com/bagder/curl/issues/318
Reported-by: sneis

lib/vtls/openssl.c

@@ -2137,10 +2137,9 @@ static CURLcode ossl_connect_step2(struct connectdata *conn, int sockindex)
     else {
       /* untreated error */
       unsigned long errdetail;
-      char error_buffer[256]; /* OpenSSL documents that this must be at least
-                                 256 bytes long. */
+      char error_buffer[256]=""; /* OpenSSL documents that this must be at
+                                    least 256 bytes long. */
       CURLcode result;
-      const char *cert_problem = NULL;
       long lerr;
 
       connssl->connecting_state = ssl_connect_2; /* the connection failed,
@@ -2172,9 +2171,10 @@ static CURLcode ossl_connect_step2(struct connectdata *conn, int sockindex)
                    X509_verify_cert_error_string(lerr));
         }
         else
-          cert_problem = "SSL certificate problem, verify that the CA cert is"
-            " OK.";
-
+          /* strcpy() is fine here as long as the string fits within
+             error_buffer */
+          strcpy(error_buffer,
+                 "SSL certificate problem, check your CA cert");
         break;
       default:
         result = CURLE_SSL_CONNECT_ERROR;
@@ -2195,7 +2195,7 @@ static CURLcode ossl_connect_step2(struct connectdata *conn, int sockindex)
       }
 
       /* Could be a CERT problem */
-      failf(data, "%s%s", cert_problem ? cert_problem : "", error_buffer);
+      failf(data, "%s", error_buffer);
 
       return result;
     }