regexec.c: No statements before declarations

Commit b99851e1941e002dd4816ee6c76fd49bbee1d7f3 moved a statement to
before the declarations in its block.  Mea culpa.  It turns out that I
missed the warnings that were shown for -W=declaration-after-statement.
I'm henceforth changing that to -Werror=declaration-after-statement.