target/riscv/riscv-011: fix compile warning about uninitialized variable

In MSYS2 MinGW 64-bit
git clone git://git.code.sf.net/p/openocd/code openocd
$ gcc --version
gcc.exe (Rev1, Built by MSYS2 project) 8.2.0
./bootstrap
./configure --prefix=
$ cat config.status | grep CFLAGS
CFLAGS='-g -O2'
make bindir = "bin-x64"

depbase=`echo src/target/riscv/riscv-011.lo | sed 's|[^/]*$|.deps/&|;s|\.lo$||'`;\
/bin/sh ./libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I.  -D__USE_MINGW_ANSI_STDIO -I./src -I./src -I./src/helper -DPKGDATADIR=\"/mingw64/share/openocd\" -DBINDIR=\"bin-x64\" -I./jimtcl -I./jimtcl  -Wall -Wstrict-prototypes -Wformat-security -Wshadow -Wextra -Wno-unused-parameter -Wbad-function-cast -Wcast-align -Wredundant-decls -Werror -g -O2 -MT src/target/riscv/riscv-011.lo -MD -MP -MF $depbase.Tpo -c -o src/target/riscv/riscv-011.lo src/target/riscv/riscv-011.c &&\
mv -f $depbase.Tpo $depbase.Plo
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -D__USE_MINGW_ANSI_STDIO -I./src -I./src -I./src/helper -DPKGDATADIR=\"/mingw64/share/openocd\" -DBINDIR=\"bin-x64\" -I./jimtcl -I./jimtcl -Wall -Wstrict-prototypes -Wformat-security -Wshadow -Wextra -Wno-unused-parameter -Wbad-function-cast -Wcast-align -Wredundant-decls -Werror -g -O2 -MT src/target/riscv/riscv-011.lo -MD -MP -MF src/target/riscv/.deps/riscv-011.Tpo -c src/target/riscv/riscv-011.c -o src/target/riscv/riscv-011.o
src/target/riscv/riscv-011.c: In function 'poll_target':
src/target/riscv/riscv-011.c:1799:6: error: 'reg' may be used uninitialized in this function [-Werror=maybe-uninitialized]
      reg_cache_set(target, reg, ((data & 0xffffffff) << 32) | value);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/target/riscv/riscv-011.c:1686:17: note: 'reg' was declared here
    unsigned int reg;
                 ^~~
cc1.exe: all warnings being treated as errors
make[2]: *** [Makefile:3250: src/target/riscv/riscv-011.lo] Error 1

Change-Id: I6996dcb866fbace26817636f4bedba09510a087f
Signed-off-by: Svetoslav Enchev <svetoslav.enchev@gmail.com>
Reviewed-on: http://openocd.zylin.com/4635
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/riscv/riscv-011.c

@@ -1788,6 +1788,8 @@ static riscv_error_t handle_halt_routine(struct target *target)
 					break;
 				default:
 					assert(0);
+					LOG_ERROR("Got invalid register result %d", result);
+					goto error;
 			}
 			if (riscv_xlen(target) == 32) {
 				reg_cache_set(target, reg, data & 0xffffffff);