Fix bug in magic module when libmagic returns null pointer.

The pointer returned from `libmagic` was passed directly to `yr_strdup` (a wrapper of `strdup`), but this function has undefined behavior when the argument is null.

libyara/modules/magic/magic.c

@@ -111,8 +111,10 @@ define_function(magic_mime_type)
     {
       magic_setflags(cache->magic_cookie, MAGIC_MIME_TYPE);
 
-      cache->cached_mime_type = yr_strdup(
-          magic_buffer(cache->magic_cookie, block_data, block->size));
+      const char* type = magic_buffer(
+          cache->magic_cookie, block_data, block->size);
+
+      cache->cached_mime_type = (type == NULL) ? NULL : yr_strdup(type);
     }
   }
 
@@ -148,8 +150,10 @@ define_function(magic_type)
     {
       magic_setflags(cache->magic_cookie, 0);
 
-      cache->cached_type = yr_strdup(
-          magic_buffer(cache->magic_cookie, block_data, block->size));
+      const char* type = magic_buffer(
+          cache->magic_cookie, block_data, block->size);
+
+      cache->cached_type = (type == NULL) ? NULL : yr_strdup(type);
     }
   }