The BEXT coding history parser allocates extraBytes+1 bytes (the +1
for a null terminator) then reads extraBytes from the file, but never
writes the null terminator. The subsequent ma_dr_wav__strlen() call
scans past the buffer into uninitialized heap memory, causing an
unbounded out-of-bounds read.

Null-terminate the buffer at the actual bytes-read position and use
the byte count directly for codingHistorySize, eliminating the unsafe
strlen call entirely.

Fixes mackron#1101