Suppress compiler warning in new regex match-all detection code.

gcc 10 is smart enough to notice that control could reach this
"hasmatch[depth]" assignment with depth < 0, but not smart enough
to know that that would require a badly broken NFA graph.  Change
the assert() to a plain runtime test to shut it up.

Per report from Andres Freund.

Discussion: https://postgr.es/m/20210223173437.b3ywijygsy6q42gq@alap3.anarazel.de

Author: tglsfdc

src/backend/regex/regc_nfa.c

@@ -3082,8 +3082,13 @@ checkmatchall_recurse(struct nfa *nfa, struct state *s,
 		{
 			/* We found an all-RAINBOW path to the post state */
 			result = true;
+			/* ... which should not be adjacent to the pre state */
+			if (depth < 0)
+			{
+				NERR(REG_ASSERT);
+				return false;
+			}
 			/* Record potential match lengths */
-			assert(depth >= 0);
 			hasmatch[depth] = true;
 			if (foundloop)
 			{