meta: fix tc classid parsing out-of-bounds access

meta: fix tc classid parsing out-of-bounds access

AddressSanitizer: heap-buffer-overflow on address 0x6020000003af ...
  #0 0x7f9a83cbb402 in tchandle_type_parse src/meta.c:89
  #1 0x7f9a83c6753f in symbol_parse src/datatype.c:138

strlen() - 1 can underflow if length was 0.

Simplify the function, there is no need to duplicate the string
while scanning it.

Expect the first strtol to stop at ':', scan for the minor number next.
The second scan is required to stop at '\0'.

Fixes: 6f2eb8548e0d ("src: meta priority support using tc classid")
Signed-off-by: Florian Westphal <fw@strlen.de>