Fix heap-buffer-overflow in pglz_decompress() on corrupt input.

When decoding a match tag, pglz_decompress() reads 2 bytes (or 3
for extended-length matches) from the source buffer before checking
whether enough data remains.  The existing bounds check (sp > srcend)
occurs after the reads, so truncated compressed data that ends
mid-tag causes a read past the allocated buffer.

Fix by validating that sufficient source bytes are available before
reading each part of the match tag.  The post-read sp > srcend
check is no longer needed and is removed.

Found by fuzz testing with libFuzzer and AddressSanitizer.

src/common/pg_lzcompress.c

@@ -727,22 +727,33 @@ pglz_decompress(const char *source, int32 slen, char *dest,
 				int32		len;
 				int32		off;
 
+				/*
+				 * A match tag is at least 2 bytes; if the length nibble is
+				 * 0x0f the tag is 3 bytes (extended length).  Verify we have
+				 * enough source data before reading them.
+				 */
+				if (unlikely(sp + 2 > srcend))
+					return -1;
+
 				len = (sp[0] & 0x0f) + 3;
 				off = ((sp[0] & 0xf0) << 4) | sp[1];
 				sp += 2;
 				if (len == 18)
+				{
+					if (unlikely(sp >= srcend))
+						return -1;
 					len += *sp++;
+				}
 
 				/*
-				 * Check for corrupt data: if we fell off the end of the
+				 * Check for corrupt data: if we obtained off = 0, or if off
-				 * source, or if we obtained off = 0, or if off is more than
+				 * is more than the distance back to the buffer start, we have
-				 * the distance back to the buffer start, we have problems.
+				 * problems.  (We must check for off = 0, else we risk an
-				 * (We must check for off = 0, else we risk an infinite loop
+				 * infinite loop below in the face of corrupt data. Likewise,
-				 * below in the face of corrupt data.  Likewise, the upper
+				 * the upper limit on off prevents accessing outside the
-				 * limit on off prevents accessing outside the buffer
+				 * buffer boundaries.)
-				 * boundaries.)
 				 */
-				if (unlikely(sp > srcend || off == 0 ||
+				if (unlikely(off == 0 ||
 							 off > (dp - (unsigned char *) dest)))
 					return -1;