colblk: fix panic in Invert

Fix a panic where Invert would pass a negative number into slices.Grow if bits
at indices higher than `nRows` have been set.

sstable/colblk/bitmap.go

@@ -338,7 +338,10 @@ func (b *BitmapBuilder) Invert(nRows int) {
 	b.minNonZeroRowCount = 1
 	// If the tail of b is sparse, fill in zeroes before inverting.
 	nBitmapWords := (nRows + 63) >> 6
-	b.words = slices.Grow(b.words, nBitmapWords-len(b.words))[:nBitmapWords]
+	if len(b.words) < nBitmapWords {
+		b.words = slices.Grow(b.words, nBitmapWords-len(b.words))
+	}
+	b.words = b.words[:nBitmapWords]
 	for i := range b.words {
 		b.words[i] = ^b.words[i]
 	}

sstable/colblk/testdata/bitmap

@@ -414,3 +414,18 @@ Binary representation:
 32-40: b 0000000000011100000000000000000000000000000000000000000000000000 # bitmap word 3
 40-48: b 0000000000000000000000000000000000000000000000000000000000000000 # bitmap word 4
 48-56: b 0000100000000000000000000000000000000000000000000000000000000000 # bitmap summary word 0-63
+
+# Test a case where we invert a bitmap that has more words than we'll need in
+# the finished serialized bitmap. Regression test for a bug that caused us to
+# pass a negative number into slices.Grow.
+
+build rows=64 invert
+0000000000000000000000000000000000000000000000000000000000000000
+1
+----
+1111111111111111111111111111111111111111111111111111111111111111
+Binary representation:
+00-01: x 00                                                               # bitmap encoding
+01-08: x 00000000000000                                                   # padding to align to 64-bit boundary
+08-16: b 1111111111111111111111111111111111111111111111111111111111111111 # bitmap word 0
+16-24: b 0000000100000000000000000000000000000000000000000000000000000000 # bitmap summary word 0-63