Silence compiler warning in assertion-enabled builds.

With -Wtype-limits, gcc correctly points out that size_t can never be < 0.
Backpatch to 9.3 and 9.2. It's been like this forever, but in <= 9.1 you got
a lot other warnings with -Wtype-limits anyway (at least with my version of
gcc).

Andres Freund

src/pl/plpython/plpy_procedure.c

@@ -493,8 +493,8 @@ PLy_procedure_munge_source(const char *name, const char *src)
 	char	   *mrc,
 			   *mp;
 	const char *sp;
-	size_t		mlen,
-				plen;
+	size_t		mlen;
+	int			plen;
 
 	/*
 	 * room for function source and the def statement