Fix a compilation warning and a typo
Add missing protection to avoid reading before the start a string.

This fixes ROOT-5353.
(cherry picked from v5-34-00-patches commit 249bf475cdf220978e428e57f95b1cfea693cba9)
Fix warning: base class ‘class TInterpreterValue’ should be explicitly initialized
Quiet down Coverity 51897, 51898 Uninitialized pointer field