Recommit r329716 "Add missing nullptr check before getSection() to...

Recommit r329716 "Add missing nullptr check before getSection() to AArch64MachObjectWriter::recordRelocation"

This commit fixes the bot failures that were coming up before with r329716.

The fix was to move the check for "isInSection()" inside of the if condition
and emit the error there instead of waiting to get past the unreachable statement.

This should work in debug and release builds now.

llvm-svn: 329746