Make sure that we don't dereference NULL

The code was actually safe, because b should
never be null, because if rvalue is empty, a different
branch is taken. But we *do* check for NULL in the
loop above, so it's better to also check here for symmetry.

src/core/load-fragment.c

@@ -1860,7 +1860,8 @@ int config_parse_documentation(const char *unit,
                         free(*a);
                 }
         }
-        *b = NULL;
+        if (b)
+                *b = NULL;
 
         return r;
 }