Fix macro-spanning if in x509_crt.c

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/x509_crt.c b/library/x509_crt.c
index d97c867..c7ee5c1 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -1258,9 +1258,12 @@
         }
     }
 
+    int extensions_allowed = 1;
 #if !defined(MBEDTLS_X509_ALLOW_EXTENSIONS_NON_V3)
-    if( crt->version == 3 )
+    if( crt->version != 3 )
+        extensions_allowed = 0;
 #endif
+    if( extensions_allowed )
     {
         ret = x509_get_crt_ext( &p, end, crt, cb, p_ctx );
         if( ret != 0 )