eliminated sign compare warning from stan/agrad/fwd/matrix/dot_product.hpp
eliminated sign compare warning from stan/agrad/fwd/matrix/dot_product.hpp

stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math