Fix incorrect BLI_snprintf usage

Event though in practice this wasn't causing problems as the fixed size
buffers are generally large enough not to truncate text.

Using the result from `snprint` or `BLI_snprintf` to step over a fixed
size buffer allows for buffer overruns as the returned value is the size
needed to copy the entire string, not the number of bytes copied.

Building strings using this convention with multiple calls:

    ofs += BLI_snprintf(str + ofs, str_len_max - ofs);

.. caused the size argument to become negative,
wrapping it to a large value when cast to the unsigned argument.

source/blender/python/mathutils/mathutils_Matrix.c

@@ -2253,7 +2253,7 @@ static PyObject *Matrix_str(MatrixObject *self)
   for (col = 0; col < self->num_col; col++) {
     maxsize[col] = 0;
     for (row = 0; row < self->num_row; row++) {
-      const int size = BLI_snprintf(
+      const int size = BLI_snprintf_rlen(
           dummy_buf, sizeof(dummy_buf), "%.4f", MATRIX_ITEM(self, row, col));
       maxsize[col] = max_ii(maxsize[col], size);
     }