tuklib_mbstr_width: Add tuklib_mbstr_width_mem()

It's a new function split from tuklib_mbstr_width().
It's useful with partial strings that aren't terminated with \0.

src/common/tuklib_mbstr.h

@@ -37,6 +37,23 @@ extern size_t tuklib_mbstr_width(const char *str, size_t *bytes);
 ///             (size_t)-1 is returned. Possible errors include invalid,
 ///             partial, or non-printable multibyte character in str.
 
+#define tuklib_mbstr_width_mem TUKLIB_SYMBOL(tuklib_mbstr_width_mem)
+extern size_t tuklib_mbstr_width_mem(const char *str, size_t len);
+///<
+/// \brief      Get the number of columns needed for the multibyte buffer
+///
+/// This is like tuklib_mbstr_width() except that this takes the buffer
+/// length in bytes as the second argument. This allows using the function
+/// for buffers that aren't terminated with '\0'.
+///
+/// \param      str         String whose width is to be calculated.
+/// \param      len         Number of bytes to read from str.
+///
+/// \return     On success, the number of columns needed to display the
+///             string e.g. in a terminal emulator is returned. On error,
+///             (size_t)-1 is returned. Possible errors include invalid,
+///             partial, or non-printable multibyte character in str.
+
 #define tuklib_mbstr_fw TUKLIB_SYMBOL(tuklib_mbstr_fw)
 extern int tuklib_mbstr_fw(const char *str, int columns_min);
 ///<

src/common/tuklib_mbstr_width.c

@@ -24,9 +24,17 @@ tuklib_mbstr_width(const char *str, size_t *bytes)
 	if (bytes != NULL)
 		*bytes = len;
 
+	return tuklib_mbstr_width_mem(str, len);
+}
+
+
+extern size_t
+tuklib_mbstr_width_mem(const char *str, size_t len)
+{
 #ifndef HAVE_MBRTOWC
 	// In single-byte mode, the width of the string is the same
 	// as its length.
+	(void)str;
 	return len;
 
 #else