KInfocenter/OpenGL: fix ReadPipe() to return 0 in the case that the command cannot be run

ReadPipe() doesn't return 0 as expected in the case that the command is not found. but the
length of sh's output which is "command not found" in this case. This is because popen()
does not fail if the command is not found, because it _can_ run "sh". (according to the
man page, popen calls "/bin/sh -c command")
To fix this, ReadPipe() should check the return code of the call to pclose()
(see "man pclose"), and return 0 if this is not equal to 0.

CCBUG: 327382
REVIEW: 113779