Allow for Magic#load to take nil as valid argument.

This will also make Magic::new accept nil, which can be handy at some
very specific occasions i.e. passing default argument, etc.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>

Author: kwilczynski

ext/magic/ruby-magic.c

@@ -83,7 +83,7 @@ static VALUE magic_return(VALUE value, void *data);
 VALUE
 rb_mgc_initialize(VALUE object, VALUE arguments)
 {
-    VALUE mutex;
+    VALUE mutex = Qnil;
 
     magic_arguments_t ma;
     const char *klass = NULL;
@@ -300,7 +300,7 @@ rb_mgc_load(VALUE object, VALUE arguments)
     CHECK_MAGIC_OPEN(object);
     MAGIC_COOKIE(ma.cookie);
 
-    if (!RARRAY_EMPTY_P(arguments)) {
+    if (!RARRAY_EMPTY_P(arguments) && !NIL_P(RARRAY_FIRST(arguments))) {
         value = magic_join(arguments, CSTR2RVAL(":"));
         ma.data.file.path = RVAL2CSTR(value);
     }

ext/magic/ruby-magic.h

@@ -58,6 +58,7 @@ extern "C" {
 
 #define RSTRING_EMPTY_P(s) (RSTRING_LEN(s) == 0)
 #define RARRAY_EMPTY_P(a)  (RARRAY_LEN(a) == 0)
+#define RARRAY_FIRST(a)    (RARRAY_EMPTY_P(a) ? Qnil : RARRAY_PTR(a)[0])
 
 #define NOGVL_FUNCTION (VALUE (*)(void *))