tools/rv: Fix curr_reactor uninitialized variable

clang is reporting:

$ make HOSTCC=clang CC=clang LLVM_IAS=1

clang -O -g -DVERSION=\"6.8.0-rc3\" -flto=auto -fexceptions
	-fstack-protector-strong -fasynchronous-unwind-tables
	-fstack-clash-protection  -Wall -Werror=format-security
	-Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS
	$(pkg-config --cflags libtracefs)  -I include
	-c -o src/in_kernel.o src/in_kernel.c
[...]

src/in_kernel.c:227:6: warning: variable 'curr_reactor' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  227 |         if (!end)
      |             ^~~~
src/in_kernel.c:242:9: note: uninitialized use occurs here
  242 |         return curr_reactor;
      |                ^~~~~~~~~~~~
src/in_kernel.c:227:2: note: remove the 'if' if its condition is always false
  227 |         if (!end)
      |         ^~~~~~~~~
  228 |                 goto out_free;
      |                 ~~~~~~~~~~~~~
src/in_kernel.c:221:6: warning: variable 'curr_reactor' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  221 |         if (!start)
      |             ^~~~~~
src/in_kernel.c:242:9: note: uninitialized use occurs here
  242 |         return curr_reactor;
      |                ^~~~~~~~~~~~
src/in_kernel.c:221:2: note: remove the 'if' if its condition is always false
  221 |         if (!start)
      |         ^~~~~~~~~~~
  222 |                 goto out_free;
      |                 ~~~~~~~~~~~~~
src/in_kernel.c:215:20: note: initialize the variable 'curr_reactor' to silence this warning
  215 |         char *curr_reactor;
      |                           ^
      |                            = NULL
2 warnings generated.

Which is correct. Setting curr_reactor to NULL avoids the problem.

Link: https://lkml.kernel.org/r/3a35551149e5ee0cb0950035afcb8082c3b5d05b.1707217097.git.bristot@kernel.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Donald Zickus <dzickus@redhat.com>
Fixes: 6d60f896 ("tools/rv: Add in-kernel monitor interface")
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>

tools/verification/rv/src/in_kernel.c

@@ -210,9 +210,9 @@ static char *ikm_read_reactor(char *monitor_name)
 static char *ikm_get_current_reactor(char *monitor_name)
 {
 	char *reactors = ikm_read_reactor(monitor_name);
+	char *curr_reactor = NULL;
 	char *start;
 	char *end;
-	char *curr_reactor;
 
 	if (!reactors)
 		return NULL;