Avoid buffer text relocations in calls to STRING_CHAR_* macros.

Avoid buffer text relocations in calls to STRING_CHAR_* macros.

 src/charset.c (maybe_unify_char): Inhibit relocation of buffer text
 for the duration of call to load_charset, to avoid problems with
 callers of maybe_unify_char that access buffer text through C
 pointers.
 src/ralloc.c (r_alloc_inhibit_buffer_relocation): Increment and
 decrement the inhibition flag, instead of just setting or
 resetting it.

Fixes: debbugs:11519