Put a cap on op slab sizes

If a subroutine is *really* big, we don’t want to allocate, say, 4MB
for ops when just over 2 will do, just because there was one op more
than could fit in 2MB.

op.c

@@ -305,6 +305,9 @@ Perl_Slab_Free(pTHX_ void *op)
 # ifndef PERL_SLAB_SIZE
 #  define PERL_SLAB_SIZE 64
 # endif
+# ifndef PERL_MAX_SLAB_SIZE
+#  define PERL_MAX_SLAB_SIZE 2048
+# endif
 
 /* rounds up to nearest pointer */
 # define SIZE_TO_PSIZE(x)	(((x) + sizeof(I32 *) - 1)/sizeof(I32 *))
@@ -391,7 +394,9 @@ Perl_Slab_Alloc(pTHX_ size_t sz)
 	/* Create a new slab.  Make this one twice as big. */
 	slot = slab2->opslab_first;
 	while (slot->opslot_next) slot = slot->opslot_next;
-	slab2 = S_new_slab(aTHX_ DIFF(slab2, slot)*2);
+	slab2 = S_new_slab(aTHX_ DIFF(slab2, slot)*2 > PERL_MAX_SLAB_SIZE
+					? PERL_MAX_SLAB_SIZE
+					: DIFF(slab2, slot)*2);
 	slab2->opslab_next = slab->opslab_next;
 	slab->opslab_next = slab2;
     }