proxy: fix trailingdata error with ascii multiget

One of the side effects of pre-warming all of the tests I did with
multiget, and not having done a second round on the unit tests, is that
we somehow never tried an ascii multiget against a damn miss.

Easy to test, easy to fix.

proxy_network.c

@@ -1037,6 +1037,11 @@ static int proxy_backend_drive_machine(mcp_backend_t *be) {
 
             if (p->ascii_multiget && r->resp.type == MCMC_RESP_END) {
                 // Ascii multiget hack mode; consume END's
+                be->rbufused -= r->resp.reslen;
+                if (be->rbufused > 0) {
+                    memmove(be->rbuf, be->rbuf+r->resp.reslen, be->rbufused);
+                }
+
                 be->state = mcp_backend_next;
                 break;
             }

t/proxyunits.t

@@ -362,6 +362,21 @@ check_version($ps);
         is(scalar <$ps>, "$key\r\n", "multiget value $key");
     }
     is(scalar <$ps>, "END\r\n", "final END from multiget");
+
+    # Test multiget workaround with misses (known bug)
+    print $ps $cmd;
+    is(scalar <$be>, "get /b/c\r\n", "multiget breakdown c");
+    is(scalar <$be>, "get /b/b\r\n", "multiget breakdown b");
+    is(scalar <$be>, "get /b/a\r\n", "multiget breakdown a");
+
+    print $be "END\r\nEND\r\nEND\r\n";
+    is(scalar <$ps>, "END\r\n", "final END from multiget");
+
+    # If bugged, the backend will have closed.
+    print $ps "get /b/a\r\n";
+    is(scalar <$be>, "get /b/a\r\n", "get works after empty multiget");
+    print $be "END\r\n";
+    is(scalar <$ps>, "END\r\n", "end after empty multiget");
 }
 
 check_version($ps);