compat/mingw.c: Fix some sparse warnings

Sparse issues the following warnings:

        SP compat/mingw.c
    compat/mingw.c:795:3: warning: symbol 'pinfo_t' was not declared. \
        Should it be static?
    compat/mingw.c:796:16: warning: symbol 'pinfo' was not declared. \
        Should it be static?
    compat/mingw.c:797:18: warning: symbol 'pinfo_cs' was not declared. \
        Should it be static?
    compat/mingw.c:1207:23: warning: Using plain integer as NULL pointer

In 'pinfo_t' variable, defined on line 795, seems to have been a
mistake (a missing typedef keyword?), so we simply remove it.

The 'pinfo' variable does not require more than file scope, so we
simply add the static modifier to the declaration.

The 'pinfo_cs' variable, in contrast, requires initialisation in the
mingw replacement main() function, so we add an extern declaration to
the compat/mingw.h header file.

The remaining warning is suppressed by replacing the rhs of the
pointer assignment with the NULL pointer literal.

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

compat/mingw.c

@@ -841,8 +841,8 @@ struct pinfo_t {
 	struct pinfo_t *next;
 	pid_t pid;
 	HANDLE proc;
-} pinfo_t;
-struct pinfo_t *pinfo = NULL;
+};
+static struct pinfo_t *pinfo = NULL;
 CRITICAL_SECTION pinfo_cs;
 
 static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **env,
@@ -1253,7 +1253,7 @@ static int WSAAPI getaddrinfo_stub(const char *node, const char *service,
 	else
 		sin->sin_addr.s_addr = INADDR_LOOPBACK;
 	ai->ai_addr = (struct sockaddr *)sin;
-	ai->ai_next = 0;
+	ai->ai_next = NULL;
 	return 0;
 }
 

compat/mingw.h

@@ -333,6 +333,13 @@ void mingw_open_html(const char *path);
 char **make_augmented_environ(const char *const *vars);
 void free_environ(char **env);
 
+/*
+ * A critical section used in the implementation of the spawn
+ * functions (mingw_spawnv[p]e()) and waitpid(). Intialised in
+ * the replacement main() macro below.
+ */
+extern CRITICAL_SECTION pinfo_cs;
+
 /*
  * A replacement of main() that ensures that argv[0] has a path
  * and that default fmode and std(in|out|err) are in binary mode