compat/regex/regexec.c: Fix some sparse warnings

compat/regex/regexec.c: Fix some sparse warnings

Sparse issues an "Using plain integer as NULL pointer" warning along
with two "symbol was not declared. Should it be static?" type warnings
for the 'merge_state_with_log' and 'find_recover_state' functions.

In order to suppress the warnings, we replace the use of '0' as a null
pointer constant with NULL and add the static modifier to the function
definitions.

Signed-off-by: Ramsay Jones <redacted>
Signed-off-by: Junio C Hamano <redacted>