Fix ConvertingString heap overflow without causing stack smash

Commit 832d8e7 changed lengthString = length/sizeof(wchar_t) to
length/sizeof(SQLWCHAR) to stop a 1-byte heap overflow in the internal
byteString when OdbcError::sqlGetDiagRec strcpy's a 6-byte SQL state
into the 5-byte buffer allocated on Linux (wchar_t=4).

That fix is wrong in the opposite direction: lengthString is also the
count passed to mbstowcs((wchar_t*)unicodeString, byteString,
lengthString) in the destructor, which writes lengthString *
sizeof(wchar_t) bytes into the caller's SQLWCHAR buffer. With
lengthString=6 and a 12-byte caller buffer, mbstowcs overruns by
12 bytes and smashes the caller's stack (reproducible without ASAN).

Revert line 88 to sizeof(wchar_t) to restore the safe mbstowcs bound,
and instead floor the internal byteString allocation at 8 bytes so the
strcpy no longer overflows. This is a targeted fix that keeps both the
ASAN job and normal runs green until the ConvertingString / mbstowcs
rewrite lands (issue #287 Tier 9.1).

Author: fdcastel

MainUnicode.cpp

@@ -85,7 +85,7 @@ class ConvertingString
 			if ( length == SQL_NTS )
 				lengthString = 0;
 			else if ( retCountOfBytes )
-				lengthString = length / sizeof(SQLWCHAR);
+				lengthString = length / sizeof(wchar_t);
 			else
 				lengthString = length;
 		}
@@ -219,8 +219,16 @@ class ConvertingString
 		case BYTESCHARS:
 			if ( lengthString )
 			{
-				byteString = new SQLCHAR[ lengthString + 2 ];
-				memset(byteString, 0, lengthString + 2); 
+				// Floor the internal buffer at 8 bytes so that callers which pass a
+				// small SQLWCHAR output buffer (e.g. SQLGetDiagRecW with a 12-byte
+				// SQL state, yielding lengthString=3 on Linux where sizeof(wchar_t)=4)
+				// still have room for the 6-byte SQL state ("HY000\0") that
+				// OdbcError::sqlGetDiagRec strcpy's into this buffer. Keeping
+				// lengthString itself unchanged preserves the mbstowcs writeback
+				// bound and avoids smashing the caller's stack buffer.
+				const size_t bufSize = (lengthString + 2 < 8) ? 8 : (size_t)lengthString + 2;
+				byteString = new SQLCHAR[ bufSize ];
+				memset(byteString, 0, bufSize);
 			}
 			else
 				byteString = NULL;