llvm-profdata: Avoid undefined behaviour when reading raw profiles

The raw profiles that are generated in compiler-rt always add padding
so that each profile is aligned, so we can simply treat files that
don't have this property as malformed.

Caught by Alexey's new ubsan bot. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217708 91177308-0d34-0410-b5e6-96231b3b80d8