Fix TestGdbRemoteLibrariesSvr4Support

D62502 had a bug (visible only with D62503 reverted), where it would
error out if attempting to read a string from memory and the memory page
after the string happened to be unmapped.

This fixes the problem by checking for whether ReadMemory read *any*
bytes, instead of checking whether it returned an error. A greater
question is whether ReadMemory should even return an error if it read at
least one byte, but I'm leaving that for a separate patch.

llvm-svn: 364748