libflux: handle flux_respond_error (errnum=0)

Problem: if flux_respond_error() is called with errnum of 0,
no response is sent, which can cause hard to diagnose problems.

Use EINVAL if errnum is zero.

Fixes flux-framework#3036

src/common/libflux/response.c

@@ -315,8 +315,10 @@ int flux_respond_error (flux_t *h, const flux_msg_t *request,
 {
     flux_msg_t *msg = NULL;
 
-    if (!h || !request || errnum == 0)
+    if (!h || !request)
         goto inval;
+    if (errnum == 0)
+        errnum = EINVAL;
     if (flux_msg_is_noresponse (request))
         return 0;
     msg = flux_response_derive (request, errnum);

src/common/libflux/response.h

@@ -86,7 +86,8 @@ int flux_respond_raw (flux_t *h, const flux_msg_t *request,
                       const void *data, int len);
 
 /* Create an error response to the provided request message with optional
- * error string payload (if errstr is non-NULL).
+ * error string payload (if errstr is non-NULL).  If errnum is zero, EINVAL
+ * is substituted.
  */
 int flux_respond_error (flux_t *h, const flux_msg_t *request,
                         int errnum, const char *errstr);