[MCJIT] Fix a case of Error::success() being passed to report_fatal_error.

MCJIT::getSymbolAddress was handling a non-fatal error condition of JITSymbol
as fatal. JITSymbol::operator bool returns false if no address is available
but no error is set. This can occur e.g. if the symbol name was not found.

Patch by Jascha Wetzel. Thanks Jascha!

llvm-svn: 339809