libluna: Fix String::from_string_view construction for inline strings

Before, this method failed to add a null terminator if the source string did not have one, which was possible.

libluna/src/String.cpp

@@ -155,7 +155,8 @@ Result<String> String::from_string_view(StringView str)
         String result;
         result.m_inline = true;
         result.m_length = str.length();
-        strncpy(result.m_inline_storage, str.chars(), sizeof(m_inline_storage));
+        memset(result.m_inline_storage, 0, sizeof(result.m_inline_storage));
+        memcpy(result.m_inline_storage, str.chars(), str.length());
         return result;
     }