patch 8.1.1025: checking NULL pointer after addition

Problem:    Checking NULL pointer after addition. (Coverity)
Solution:   First check for NULL, then add the column.
diff --git a/src/regexp.c b/src/regexp.c
index 4b85317..5557987 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -7784,9 +7784,10 @@
 	    if (lnum < 0 || rsm.sm_mmatch->endpos[no].lnum < 0)
 		return NULL;
 
-	    s = reg_getline_submatch(lnum) + rsm.sm_mmatch->startpos[no].col;
-	    if (s == NULL)  /* anti-crash check, cannot happen? */
+	    s = reg_getline_submatch(lnum);
+	    if (s == NULL)  // anti-crash check, cannot happen?
 		break;
+	    s += rsm.sm_mmatch->startpos[no].col;
 	    if (rsm.sm_mmatch->endpos[no].lnum == lnum)
 	    {
 		/* Within one line: take form start to end col. */