Address user reported coverity issues.

library/base64.c

diff --git a/library/base64.c b/library/base64.c
index 3432e5f..5cb12cb 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -97,7 +97,7 @@
 
     n *= 4;
 
-    if( dlen < n + 1 )
+    if( ( dlen < n + 1 ) || ( NULL == dst ) )
     {
         *olen = n + 1;
         return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );