Don't crash on invalid variable names in RPN

Don't crash on invalid variable names in RPN

When rpn_parse() finds [^\0,] after parsing a token, it returns NULL
without setting an error. This causes rrd_test_error() to return false
and subsequent code will dereference NULL (cf. rrdtool xport
CDEF:foo=foo-bar).

This commit changes the OP_VARIABLE branch in rpn_parse so that in order
to be a variable name, sscanf needs to match the full name, causing a
more meaningful "ERROR: don't understand 'illegal-variable-name'" error
message in that case. Also, I made the return NULL branch set an error
message so rrd_test_error() will succeed.