Initializes an uninitilized variable in md_analyze_emph Fixes the following, reported by clang analysis: src/md4c.c:3729:61: warning: variable 'opener_index' may be uninitialized when used here [-Wconditional-uninitialized] MD_MARKCHAIN* opener_chain = md_mark_chain(ctx, opener_index); ^~~~~~~~~~~~ src/md4c.c:3686:25: note: initialize the variable 'opener_index' to silence this warning int opener_index; ^ = 0