Fix possible UB in mbedtls_asn1_write_raw_buffer()

This is mostly unrelated to other commits in this PR, except for the
fact that one of the added X.509 tests revealed that with UBSan.

Signed-off-by: Manuel Pégourié-Gonnard <[email protected]>

ChangeLog.d/fix-asn1write-raw-buffer.txt

@@ -0,0 +1,5 @@
+Bugfix
+   * When calling mbedtls_asn1_write_raw_buffer() with NULL, 0 as the last two
+     arguments, undefined behaviour would be triggered, in the form of a call to
+     memcpy(..., NULL, 0). This was harmless in practice, but could trigger
+     complains from sanitizers or static analyzers.

library/asn1write.c

@@ -90,7 +90,9 @@ int mbedtls_asn1_write_raw_buffer(unsigned char **p, const unsigned char *start,
 
     len = size;
     (*p) -= len;
-    memcpy(*p, buf, len);
+    if (len != 0) {
+        memcpy(*p, buf, len);
+    }
 
     return (int) len;
 }