libctf: Fix an out-of-bounds read in ctf_lookup_by_name()

When prefixes such as struct, union, etc. are compared with the current
type (e.g. struct foo), a comparison is made with the prefix. The code
currently assumes that every type is a valid C type with a prefix,
however at times, garbage ends up in this function causing an
unpredictable crash with DTrace due to the isspace(*p) call or
subsequent calls. An example that I've seen of this is the letter 's'
being passed in, comparing true with struct as the comparison size was
(q - p) == 1, but then we increment p with the length of "struct",
resulting in an out of bounds read.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29435