gopher: remove check for path == NULL

Since it can't be NULL and it makes Coverity believe we lack proper NULL
checks. Verified by test 659, landed in commit 15401fa886.

Pointed out by Coverity CID 1442746.

Assisted-by: Dan Fandrich
Fixes #3617
Closes #3642

lib/gopher.c

@@ -88,7 +88,10 @@ static CURLcode gopher_do(struct connectdata *conn, bool *done)
 
   *done = TRUE; /* unconditionally */
 
-  if(path && query)
+  /* path is guaranteed non-NULL */
+  DEBUGASSERT(path);
+
+  if(query)
     gopherpath = aprintf("%s?%s", path, query);
   else
     gopherpath = strdup(path);