Fixed warnings for this to build cleanly with gcc 7.3.0.

[jasiek]

This fixes the following:

cc  -I/mqtt/mosquitto/src/ -I/mqtt/mosquitto/lib/ -fPIC -Wall -Werror  -DBE_CDB -DBE_POSTGRES -DBE_HTTP -DBE_JWT -DBE_FILES  -Icontrib/tinycdb-0.78// -I`pg_config --includedir` -I/src -DDEBUG=1 -I/mqtt/openssl/include   -c -o be-postgres.o be-postgres.c
be-postgres.c: In function 'addKeyValue':
be-postgres.c:392:25: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
  while (*(keywords + n) != '\0')
                         ^~
be-postgres.c:392:9: note: did you mean to dereference the pointer?
  while (*(keywords + n) != '\0')
         ^
be-postgres.c:404:19: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
  if ((keywords[n] != '\0') || (values[n] != '\0')){
                   ^~
be-postgres.c:404:7: note: did you mean to dereference the pointer?
  if ((keywords[n] != '\0') || (values[n] != '\0')){
       ^
be-postgres.c:404:42: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
  if ((keywords[n] != '\0') || (values[n] != '\0')){
                                          ^~
be-postgres.c:404:32: note: did you mean to dereference the pointer?
  if ((keywords[n] != '\0') || (values[n] != '\0')){
                                ^
cc1: all warnings being treated as errors
<builtin>: recipe for target 'be-postgres.o' failed

[jpmens]

Merged, thanks.

[jasiek]

I just realised there must be more of these in the remaining modules. PR in progress.

[jasiek]

Actually - no other warnings were found, except for the memcached backend, which I'm unsure if is entirely operational.

[jpmens]

I don't really think anybody uses that, but if you feel like silencing warnings, please feel free -- I'll accept anyway.