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 <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/regex/regexec.c

@@ -2313,7 +2313,7 @@ transit_state (reg_errcode_t *err, re_match_context_t *mctx,
 }
 
 /* Update the state_log if we need */
-re_dfastate_t *
+static re_dfastate_t *
 internal_function
 merge_state_with_log (reg_errcode_t *err, re_match_context_t *mctx,
 		      re_dfastate_t *next_state)
@@ -2326,7 +2326,7 @@ merge_state_with_log (reg_errcode_t *err, re_match_context_t *mctx,
       mctx->state_log[cur_idx] = next_state;
       mctx->state_log_top = cur_idx;
     }
-  else if (mctx->state_log[cur_idx] == 0)
+  else if (mctx->state_log[cur_idx] == NULL)
     {
       mctx->state_log[cur_idx] = next_state;
     }
@@ -2392,7 +2392,7 @@ merge_state_with_log (reg_errcode_t *err, re_match_context_t *mctx,
 /* Skip bytes in the input that correspond to part of a
    multi-byte match, then look in the log for a state
    from which to restart matching.  */
-re_dfastate_t *
+static re_dfastate_t *
 internal_function
 find_recover_state (reg_errcode_t *err, re_match_context_t *mctx)
 {