Remove some superfluous NULL checks

In all of these situations, NULL is logically not allowed, making the
checks redundant.

Coverity complained about accessing the pointers before checking them
for NULL later.

player/configfiles.c

@@ -268,7 +268,7 @@ static bool needs_config_quoting(const char *s)
 {
     if (s[0] == '%')
         return true;
-    for (int i = 0; s && s[i]; i++) {
+    for (int i = 0; s[i]; i++) {
         unsigned char c = s[i];
         if (!mp_isprint(c) || mp_isspace(c) || c == '#' || c == '\'' || c == '"')
             return true;