lib: ofp-parse segfaults if required argument isn't specified

Running ovs-ofctl add-flow br0 "in_port=3 actions=resubmit" would
segfault instead of reporting an error.

lib/ofp-parse.c

@@ -40,6 +40,10 @@ str_to_u32(const char *str)
     char *tail;
     uint32_t value;
 
+    if (!str) {
+        ovs_fatal(0, "missing required numeric argument");
+    }
+
     errno = 0;
     value = strtoul(str, &tail, 0);
     if (errno == EINVAL || errno == ERANGE || *tail) {