Passing NULL to memcpy is UB.

UBSan warns when passing a NULL as the src
argument to memcpy. Avoid doing that.

src/compiler/codegen_c.c

@@ -164,8 +164,13 @@ int fb_copy_scope(fb_scope_t *scope, char *buf)
         buf[0] = '\0';
         return -1;
     }
+
     len = (size_t)scope->prefix.len;
-    memcpy(buf, scope->prefix.s, len);
+    if (scope->prefix.s != NULL) {
+        /* Passing NULL as the 2nd argument to memcpy yields undefined behaviour. */
+        memcpy(buf, scope->prefix.s, len);
+    }
+
     for (name = scope->name; name; name = name->link) {
         n = (size_t)name->ident->len;
         memcpy(buf + len, name->ident->text, n);