Do not segfault if cannot find chip in config files

stlink_chipid_get_params() used to segfault on memcmp() when
struct stlink_chipid_params *params was NULL. This could happen if
either:
- there were no chip config files (*.chip), or
- process_chipfile() failed to parse chip_id from the chip config files.
The latter case is caused by the usage of atoi() to parse the chip id.
Since the chip id is stored in hex, atoi() returns 0; such id cannot be
matched to any actual chip.

The segfault occurs on commit a52e1bc,
in file src/stlink-lib/chipid.c:957
(https://github.com/stlink-org/stlink/blob/a52e1bc5489e23f3c1071c6912820efacaa3b22c/src/stlink-lib/chipid.c#L957).

Check if params is NULL, in such case, set it to p2, which should not be
NULL as long as struct stlink_chipid_params devices[] exists.

May fix (workaround) #1163.

src/stlink-lib/chipid.c

@@ -954,7 +954,9 @@ struct stlink_chipid_params *stlink_chipid_get_params(uint32_t chipid) {
   p2 = stlink_chipid_get_params_old(chipid);
 
 #if 1
-  if (memcmp (p2, params, sizeof (struct stlink_chipid_params) - sizeof (struct stlink_chipid_params *)) != 0) {
+  if (params == NULL) {
+    params = p2;
+  } else if (memcmp (p2, params, sizeof (struct stlink_chipid_params) - sizeof (struct stlink_chipid_params *)) != 0) {
     //fprintf (stderr, "Error, chipid params not identical\n");
     //return NULL;
     fprintf(stderr, "---------- old ------------\n");