Be careful about the lifetime of stack variables

Be careful about the lifetime of stack variables

This commit fixes minitube hanging indefinitely on OpenBSD while showing search results.

ptr is pointing to free'd memory, causing an infinite loop when parsing video duration property into src/paginatedvideosource.cpp (L148)

See also the following links for a better explanation:

http://blog.aaronballman.com/2011/07/returning-stack-based-values/

https://wiki.qt.io/Qt_project_org_faq#How_can_I_convert_a_QString_to_char.2A_and_vice_versa.3F