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

[sas]

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 <[e-mail]>

[sas]

This seems like a fairly straightforward fix so I'll merge it.

lldb/trunk/source/Plugins/Language/ObjC/ObjCLanguage.cpp

bool ObjCLanguage::MethodName::SetName(llvm::StringRef name, bool strict) {
   if (name.empty())
     return IsValid(strict);
 
   // If "strict" is true. then the method must be specified with a
   // '+' or '-' at the beginning. If "strict" is false, then the '+'
   // or '-' can be omitted
   bool valid_prefix = false;
 
-  if (name[0] == '+' || name[0] == '-') {
+  if (name.size() > 1 && (name[0] == '+' || name[0] == '-')) {
     valid_prefix = name[1] == '[';
     if (name[0] == '+')
       m_type = eTypeClassMethod;
     else
       m_type = eTypeInstanceMethod;
   } else if (!strict) {
     // "strict" is false, the name just needs to start with '['
     valid_prefix = name[0] == '[';