Fix two bugs in TGParser::ParseValue

TGParser::ParseValue contains two recursive calls, one to parse the RHS of a list paste operator and one to parse the RHS of a paste operator in a class/def name. Both of these calls neglect to check the return value to see if it is null (because of some error). This causes a crash in the next line of code, which uses the return value. The code now checks for null returns.

Differential Revision: https://reviews.llvm.org/D85852