-Wmemsize-comparison warning on clang

[Jack Howarth]

The gdb-7.9 release produces a warning on clang for -Wmemsize-comparison of...

remote.c:5519:47: warning: size argument in 'strncmp' call is a
comparison [-Wmemsize-comparison]
              && strncmp (p, "core", strlen ("core") != 0))
                                     ~~~~~~~~~~~~~~~~^~~~
remote.c:5519:11: note: did you mean to compare the result of 'strncmp' instead?
              && strncmp (p, "core", strlen ("core") != 0))
                 ^                                       ~
                                                    )
remote.c:5519:31: note: explicitly cast the argument to size_t to
silence this warning
              && strncmp (p, "core", strlen ("core") != 0))
                                     ^
                                     (size_t)(           )