ingenic: check the fopen before writing the sensor clock enable (#209)

ingenic_enable_sensor_clock() opened /proc/jz/clock/cgu_cim/enable for writing
and passed the result straight to fprintf() without looking at it. The node is
0600 and the open can fail -- not running as root, or a kernel that does not
offer it for writing -- so the result was a null dereference where the honest
outcome is a probe that finds no sensor.

Reachable for as long as the function has existed, and more so since 449ce2c
made the probe path re-arm the sensor clock before every attempt rather than
once per run: the same null is now reached up to seven times per invocation.

Not demonstrated on hardware. The lab T31L reads `enabled` from that node, so
the branch holding the fopen is not taken there, and engineering a failure would
mean gating the sensor clock off underneath a running streamer. Verified on it
that detection is unchanged: SC2332 on bus 0, before and after.

Found while verifying 449ce2c's sensor-bus fix on that board, which also
confirmed the Ingenic half of it: with /dev/i2c-3 pointing at adapter 0 and
/dev/i2c-0 moved aside, the released build reports no sensor and current master
reports SC2332 on bus 3 -- exercising both the gap-tolerant sweep and
ingenic_open_i2c_fd() honouring its adapter argument, which it previously
ignored.

Author: openipc-ai

src/hal/ingenic.c

@@ -352,6 +352,14 @@ static void ingenic_enable_sensor_clock() {
                        sizeof(buf))) {
         if (!strcmp(buf, "disabled")) {
             f = fopen("/proc/jz/clock/cgu_cim/enable", "w");
+            /* The file is readable at 0600 and we may not be root, or the
+             * kernel may not offer it for writing at all. Unchecked, this was
+             * a null fprintf() -- a segfault where the honest outcome is a
+             * probe that finds no sensor. It matters more since the probe path
+             * began re-arming the clock before every attempt rather than once
+             * per run: the same null is now reached up to seven times. */
+            if (!f)
+                return;
             fprintf(f, "%s", "1");
             uint32_t direct;
             mem_reg(0x10010030, &direct, OP_READ);