[RuntimeDyld][MachO] Fix handling of empty eh-frame sections.

This patch switches from an unguarded to a guarded loop for eh-frame record
fixups. In the unguarded version we would always make at least one call to
processFDE, which would then crash trying to fix up a frame that didn't exist.

Fixes <rdar://problem/24301582>

llvm-svn: 259103