Fix warnings: regexp.c: match_with_backref(): Nonnull violation: FP.

Problem: Argument with 'nonnull' attribute passed null @ 5632.
         http://neovim.org/doc/reports/clang/report-041a0e.html#EndPath.

Diagnostic: False positive.
Rationale : `p = reg_getline(clnum)` above should not be null, because
            `clnum` starts at `start_lnum` and only increments after
            that.
Resolution: Assert p not null.

src/nvim/regexp.c

@@ -43,6 +43,7 @@
 /* #undef REGEXP_DEBUG */
 /* #define REGEXP_DEBUG */
 
+#include <assert.h>
 #include <inttypes.h>
 #include <stdbool.h>
 #include <string.h>
@@ -5625,6 +5626,8 @@ static int match_with_backref(linenr_T start_lnum, colnr_T start_col, linenr_T e
 
     /* Get the line to compare with. */
     p = reg_getline(clnum);
+    assert(p);
+
     if (clnum == end_lnum)
       len = end_col - ccol;
     else