Fix Fload dangling pointer

* src/lread.c (Fload): Expand decl’s lifetime to match its use.
Bug found by gcc -fsanitize=address.
1 job for master in 58 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #1433


Name Stage Failure
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1