fixup! Allow overriding the home directory via the HOME variable

Oh wow. Cygwin's `sys_wcstombs()` has a subtle bug in that if you use
the 3-parameter version (i.e. passing an implicit (size_t)-1 as the
number of input characters), it will return the number of output bytes
*including* the NUL character (disagreeing with the documentation).

As a consequence, the previous way to expand $HOMEDRIVE$HOMEPATH into
a single buffer managed to expand *only* $HOMEDRIVE.

This fixes git-for-windows/git#414 and
git-for-windows/git#434.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/uinfo.cc

@@ -982,7 +982,7 @@ fetch_env(LPCWSTR key, char *buf, size_t size)
   if (!len || len >= max)
     return 0;
 
-  len = sys_wcstombs (buf, size, wbuf);
+  len = sys_wcstombs (buf, size, wbuf, len);
   return len && len < size ? len : 0;
 }