Fixed an assert() exposed by fuzzing. Now, instead of assert when an invalid

Fixed an assert() exposed by fuzzing.  Now, instead of assert when an invalid
instruction encoding is encountered, we just return a NULL ARMBasicMCBuilder
instance and the client just returns false to indicate disassembly error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101201 91177308-0d34-0410-b5e6-96231b3b80d8