Fix warnings: regexp.c: skip_regexp: Np dereference: FP.

Problem: Derefence of null pointer @ 1208.
         http://neovim.org/doc/reports/clang/report-24b5ca.html#Path10

Diagnostic: False positive.
Rationale : Error is reported to happen  if after `if (*newp == NULL) {`
            body, `*newp` continues being NULL, and false branch of
            following `if (*newp != NULL)` is taken. Now, `vim_strsave`
            cannot return NULL, so error cannot happen.
Resolution: Remove dead code (leftover since OOM refactors).

src/nvim/regexp.c

@@ -1199,10 +1199,7 @@ char_u *skip_regexp(char_u *startp, int dirc, int magic, char_u **newp)
           *newp = vim_strsave(startp);
           p = *newp + (p - startp);
         }
-        if (*newp != NULL)
-          STRMOVE(p, p + 1);
-        else
-          ++p;
+        STRMOVE(p, p + 1);
       } else
         ++p;            /* skip next character */
       if (*p == 'v')