devel/llvm10: really fix the AT_EXECPATH fallback =(

It turns out that I made not one but *two* mistakes in applying this patch
from the version that I had sent upstream. This one is not an extra
semicolon, but rather not dereferencing p... p may never hit AT_NULL, but *p
certainly will.

The problem was isolated and the fix identified + tested by Andrew Gierth.

Reported by: Andrew Gierth <andrew_tao173 riddles org uk>
MFH: 2020Q4 (blanket: build fix)