Fix wrong index when writing to array

[michael-hartmann]

This commit fixes a bug: At one point the index when writing to the array rlist
was wrong. The original QUADPACK code looks like:

80   small = 0.375d+00
     erlarg = errsum
     ertest = errbnd
     rlist2(2) = area
90 continue

As arrays start with index 0 in C, the access of rlist2 should look like:

rlist[1] = area;

This change also fixes undeterministic behavior of the function dquadi due to a
use of uninitialized value. With this fix the memory access seems fine and
LLVM's MemorySanitizer does no longer complain.

[edisongustavo]

Nice, good catch!