config: handle NULL value when parsing non-bools

When the config parser sees an "implicit" bool like:

  [core]
  someVariable

it passes NULL to the config callback. Any callback code which expects a
string must check for NULL. This usually happens via helpers like
git_config_string(), etc, but some custom code forgets to do so and will
segfault.

These are all fairly vanilla cases where the solution is just the usual
pattern of:

  if (!value)
        return config_error_nonbool(var);

though note that in a few cases we have to split initializers like:

  int some_var = initializer();

into:

  int some_var;
  if (!value)
        return config_error_nonbool(var);
  some_var = initializer();

There are still some broken instances after this patch, which I'll
address on their own in individual patches after this one.

Reported-by: Carlos Andrés Ramírez Cataño <antaigroupltda@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -748,6 +748,8 @@ static int git_blame_config(const char *var, const char *value,
 	}
 
 	if (!strcmp(var, "blame.coloring")) {
+		if (!value)
+			return config_error_nonbool(var);
 		if (!strcmp(value, "repeatedLines")) {
 			coloring_mode |= OUTPUT_COLOR_LINE;
 		} else if (!strcmp(value, "highlightRecent")) {

builtin/checkout.c

@@ -1202,6 +1202,8 @@ static int git_checkout_config(const char *var, const char *value,
 	struct checkout_opts *opts = cb;
 
 	if (!strcmp(var, "diff.ignoresubmodules")) {
+		if (!value)
+			return config_error_nonbool(var);
 		handle_ignore_submodules_arg(&opts->diff_options, value);
 		return 0;
 	}

builtin/clone.c

@@ -791,6 +791,8 @@ static int git_clone_config(const char *k, const char *v,
 			    const struct config_context *ctx, void *cb)
 {
 	if (!strcmp(k, "clone.defaultremotename")) {
+		if (!v)
+			return config_error_nonbool(k);
 		free(remote_name);
 		remote_name = xstrdup(v);
 	}

builtin/log.c

@@ -594,8 +594,11 @@ static int git_log_config(const char *var, const char *value,
 			decoration_style = 0; /* maybe warn? */
 		return 0;
 	}
-	if (!strcmp(var, "log.diffmerges"))
+	if (!strcmp(var, "log.diffmerges")) {
+		if (!value)
+			return config_error_nonbool(var);
 		return diff_merges_config(value);
+	}
 	if (!strcmp(var, "log.showroot")) {
 		default_show_root = git_config_bool(var, value);
 		return 0;

builtin/pack-objects.c

@@ -3204,7 +3204,7 @@ static int git_pack_config(const char *k, const char *v,
 		return 0;
 	}
 	if (!strcmp(k, "uploadpack.blobpackfileuri")) {
-		struct configured_exclusion *ex = xmalloc(sizeof(*ex));
+		struct configured_exclusion *ex;
 		const char *oid_end, *pack_end;
 		/*
 		 * Stores the pack hash. This is not a true object ID, but is
@@ -3212,6 +3212,10 @@ static int git_pack_config(const char *k, const char *v,
 		 */
 		struct object_id pack_hash;
 
+		if (!v)
+			return config_error_nonbool(k);
+
+		ex = xmalloc(sizeof(*ex));
 		if (parse_oid_hex(v, &ex->e.oid, &oid_end) ||
 		    *oid_end != ' ' ||
 		    parse_oid_hex(oid_end + 1, &pack_hash, &pack_end) ||