Fix math source span

[toothache]

Fix math source span calculation.

The span is wrong if a math is followed with more texts:

Check("0\n012 \$abcd\$ 321", @"
paragraph ( 0, 0) 0-15
literal ( 0, 0) 0-0
linebreak ( 0, 1) 1-1
literal ( 1, 0) 2-5
math ( 1, 4) 6-15 (incorrect)
attributes ( 0, 0) 0--1
literal ( 1,10) 12-15
", "mathematics");