core: check that string pointer is not NULL in function "string_shared_get"

src/core/wee-string.c

@@ -3616,6 +3616,9 @@ string_shared_get (const char *string)
     char *key;
     int length;
 
+    if (!string)
+        return NULL;
+
     if (!string_hashtable_shared)
     {
         /*

tests/unit/core/test-core-string.cpp

@@ -1839,7 +1839,10 @@ TEST(CoreString, Shared)
     const char *str1, *str2, *str3;
     int count;
 
-    count = string_hashtable_shared->items_count;
+    count = (string_hashtable_shared) ?
+        string_hashtable_shared->items_count : 0;
+
+    POINTERS_EQUAL(NULL, string_shared_get (NULL));
 
     str1 = string_shared_get ("this is a test");
     CHECK(str1);