ccsrch.c: eliminate comparison against str literal

The clang C compiler produces the following warning.

ccsrch.c:828:26: warning: result of comparison against a string literal is
      unspecified (use strncmp instead) [-Wstring-compare]
                if (ext != NULL && ext != "")
                                       ^  ~~

This patch corrects the issue by using another way of checking for
an empty string, checking the 0th element of ext for the null byte.
Merge pull request #15 from tcort/warnings

ccsrch.c: eliminate comparison against str literal