utf8: automatically determine length of string if SIZE_MAX is specified

Let's make utf8_to_utf16() and utf16_to_utf8() a bit nicer to use by
adding shortcuts for common cases.

This is particularly relevant for utf16_to_utf8() since the
multiplication with 2 is easy to forget.

src/basic/efivars.c

@@ -265,7 +265,7 @@ finish:
 int efi_set_variable_string(const char *variable, const char *value) {
         _cleanup_free_ char16_t *u16 = NULL;
 
-        u16 = utf8_to_utf16(value, strlen(value));
+        u16 = utf8_to_utf16(value, SIZE_MAX);
         if (!u16)
                 return -ENOMEM;
 

src/basic/utf8.c

@@ -389,11 +389,23 @@ char *utf16_to_utf8(const char16_t *s, size_t length /* bytes! */) {
         const uint8_t *f;
         char *r, *t;
 
+        if (length == 0)
+                return new0(char, 1);
+
         assert(s);
 
+        if (length == SIZE_MAX) {
+                length = char16_strlen(s);
+
+                if (length > SIZE_MAX/2)
+                        return NULL; /* overflow */
+
+                length *= 2;
+        }
+
         /* Input length is in bytes, i.e. the shortest possible character takes 2 bytes. Each unicode character may
          * take up to 4 bytes in UTF-8. Let's also account for a trailing NUL byte. */
-        if (length * 2 < length)
+        if (length > (SIZE_MAX - 1) / 2)
                 return NULL; /* overflow */
 
         r = new(char, length * 2 + 1);
@@ -463,8 +475,17 @@ char16_t *utf8_to_utf16(const char *s, size_t length) {
         char16_t *n, *p;
         int r;
 
+        if (length == 0)
+                return new0(char16_t, 1);
+
         assert(s);
 
+        if (length == SIZE_MAX)
+                length = strlen(s);
+
+        if (length > SIZE_MAX - 1)
+                return NULL; /* overflow */
+
         n = new(char16_t, length + 1);
         if (!n)
                 return NULL;