[FileCheck] Fix parsing empty global and pseudo variable names

In `Pattern::parseVariable`, for global variables (those starting with
'$') and for pseudo variables (those starting with '@') the first
character is consumed before actual variable name parsing. If the name
is empty, it leads to out-of-bound access to the corresponding
`StringRef`.

This patch adds an if statement against the case described.

Author: kovdan01

llvm/lib/FileCheck/FileCheck.cpp

@@ -297,6 +297,12 @@ Pattern::parseVariable(StringRef &Str, const SourceMgr &SM) {
   if (Str[0] == '$' || IsPseudo)
     ++I;
 
+  if (I == Str.size())
+    return ErrorDiagnostic::get(SM, Str,
+                                StringRef("empty ") +
+                                    (IsPseudo ? "pseudo " : "global ") +
+                                    "variable name");
+
   if (!isValidVarNameStart(Str[I++]))
     return ErrorDiagnostic::get(SM, Str, "invalid variable name");