Avoid invalid string access in ObjCLanguage::MethodName::SetName

Summary:
Don't access `name[1]  if the string is only of length 1.  Avoids a
crash/assertion failure when parsing the string `-`.

Test Plan:
Debug a swift binary, set a breakpoint, watch lldb not crash

Original change by Paul Menage <menage@fb.com>

Reviewers: lldb-commits, clayborg

Differential Revision: https://reviews.llvm.org/D33853

llvm-svn: 304725