Fix bug reported by Valgrind. Reason: m_metavars is a vector of metavar_info; each metavar_info has a context; when we invoke mk_metavar the vector can grow in size, and the context is moved to a new location. The previous location is invalidated. To avoid the problem we have to save ctx in a local variable.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>