[PATCH] RISC-V: fix nullptr dereference in parse_arch

[PATCH] RISC-V: fix nullptr dereference in parse_arch

When parsing target attributes, if an invalid architecture string is
provided, the function parse_single_ext may return nullptr. The existing
code does not check for this case, leading to a nullptr dereference when
attempting to access the returned pointer. This patch adds a check to
ensure that the returned pointer is not nullptr before dereferencing it.
If it is nullptr, an appropriate error message is generated.

gcc/ChangeLog:

	* config/riscv/riscv-target-attr.cc
	(riscv_target_attr_parser::parse_arch): Fix nullptr dereference
	when parsing invalid arch string.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/target-attr-bad-11.c: New test.