[AArch64][AsmParser] Fix segfault on illegal fpimm.

Floating point immediate combining a negative sign and
a hexadecimal number, e.g. #-0x0  caused the compiler to crash.

Reviewers: rengolin, fhahn, samparker, SjoerdMeijer, javed.absar

Reviewed By: javed.absar

Differential Revision: https://reviews.llvm.org/D47483

llvm-svn: 333524