Mortgage rates slid this week as worries about the economy made mortgage-backed securities that fund most mortgage loans look like a safe bet to investors.Freddie Mac's weekly Primary Mortgage Market Survey showed rates on 15-year fixed-rate loans hitting a new all-time low, and rates for the workhorse 30-year fixed-rate mortgage just above the record low.Rates on 30-year fixed-rate mortgages averaged 3.88 percent with an average 0.7 point for the week ending April 12, down from 3.98 percent last week and 4.91 percent a year ago. Rates on 30-year fixed-rate mortgages hit an all-time low in records dating to 1971 of 3.87 percent during the first three weeks of February.For 15-year fixed-rate mortgages, rates averaged 3.11 percent with an average 0.7 point, down from 3.21 percent last week a...