convert: fix handling of dashless UTF prefix in validate_encoding()

Strip "UTF" and an optional dash from the start of 'upper' without
passing a NULL pointer to skip_prefix() in the second call, as it cannot
handle that.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

convert.c

@@ -289,8 +289,8 @@ static int validate_encoding(const char *path, const char *enc,
 			const char *stripped = NULL;
 			char *upper = xstrdup_toupper(enc);
 			upper[strlen(upper)-2] = '\0';
-			if (!skip_prefix(upper, "UTF-", &stripped))
-				skip_prefix(stripped, "UTF", &stripped);
+			if (skip_prefix(upper, "UTF", &stripped))
+				skip_prefix(stripped, "-", &stripped);
 			advise(advise_msg, path, stripped);
 			free(upper);
 			if (die_on_error)
@@ -309,8 +309,8 @@ static int validate_encoding(const char *path, const char *enc,
 				"working-tree-encoding.");
 			const char *stripped = NULL;
 			char *upper = xstrdup_toupper(enc);
-			if (!skip_prefix(upper, "UTF-", &stripped))
-				skip_prefix(stripped, "UTF", &stripped);
+			if (skip_prefix(upper, "UTF", &stripped))
+				skip_prefix(stripped, "-", &stripped);
 			advise(advise_msg, path, stripped, stripped);
 			free(upper);
 			if (die_on_error)