Suppressed spurious compilation warning with gcc-7 and older.

src/njs_regexp.c:1335:19: error: ‘pos’ may be used uninitialized in this
function [-Werror=maybe-uninitialized]
    pos = njs_string_offset(s.start, s.start + s.size, pos) - s.start;
diff --git a/src/njs_regexp.c b/src/njs_regexp.c
index 00a73e7..c96a53a 100644
--- a/src/njs_regexp.c
+++ b/src/njs_regexp.c
@@ -1301,6 +1301,7 @@
     }
 
     i = 0;
+    pos = 0;
     next_pos = 0;
 
     while (i < results.items) {