Curl_saferealloc: Fixed typo in docblock

Closes #3029

lib/strdup.c

@@ -81,7 +81,7 @@ void *Curl_memdup(const void *src, size_t length)
  * Curl_saferealloc(ptr, size)
  *
  * Does a normal realloc(), but will free the data pointer if the realloc
- * fails. If 'size' is zero, it will free the data and return a failure.
+ * fails. If 'size' is non-zero, it will free the data and return a failure.
  *
  * This convenience function is provided and used to help us avoid a common
  * mistake pattern when we could pass in a zero, catch the NULL return and end