[RISCV] Fix a potential issue in shouldInsertFixupForCodeAlign()

The bool result of shouldInsertExtraNopBytesForCodeAlign() is not checked but
the returned nop count is unconditionally read even though it could be
uninitialized.

Differential Revision: https://reviews.llvm.org/D63285
Patch by Edward Jones.

llvm-svn: 366175