Fix SBFrame::FindValue for when only global variables exist.

Summary:
sc.block->AppendVariables(...) returns 0 if there are no arguments or local
variables, but we still need to check for global variables.

Test Plan:
```
$ cat test.cpp
int i;
int main() {
}
$ lldb test -o 'b main' -o r
(lldb) script
>>> print lldb.frame.FindValue('i', lldb.eValueTypeVariableGlobal)
(int) i = 0 # as opposed to "No value"
```

Reviewers: jingham, ovyalov, vharron, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8464

llvm-svn: 232767