Do not pass invalid ISO-8859-{3,6,7,8} characters through silently

mbstring has a bad habit of passing invalid characters through silently
when converting to the same (or a "compatible") encoding.

For example, if you give it an invalid JIS X 0208 kuten code encoded with SJIS,
and try to convert that to EUC-JP, mbstring will just quietly re-encode the
invalid code in the EUC-JP representation.

At the same, some parts of the code (like `mb_check_encoding`) assume that
invalid characters will be treated as... well, invalid. Let's unbreak things
by actually catching errors and reporting them, instead of swallowing them.

Author: alexdowad

ext/mbstring/libmbfl/filters/mbfilter_iso8859_3.c

@@ -121,9 +121,6 @@ int mbfl_filt_conv_wchar_8859_3(int c, mbfl_convert_filter *filter)
 			}
 			n--;
 		}
-		if (s <= 0 && (c & ~MBFL_WCSPLANE_MASK) == MBFL_WCSPLANE_8859_3) {
-			s = c & MBFL_WCSPLANE_MASK;
-		}
 	}
 
 	if (s >= 0) {

ext/mbstring/libmbfl/filters/mbfilter_iso8859_6.c

@@ -121,9 +121,6 @@ int mbfl_filt_conv_wchar_8859_6(int c, mbfl_convert_filter *filter)
 			}
 			n--;
 		}
-		if (s <= 0 && (c & ~MBFL_WCSPLANE_MASK) == MBFL_WCSPLANE_8859_6) {
-			s = c & MBFL_WCSPLANE_MASK;
-		}
 	}
 
 	if (s >= 0) {

ext/mbstring/libmbfl/filters/mbfilter_iso8859_7.c

@@ -121,9 +121,6 @@ int mbfl_filt_conv_wchar_8859_7(int c, mbfl_convert_filter *filter)
 			}
 			n--;
 		}
-		if (s <= 0 && (c & ~MBFL_WCSPLANE_MASK) == MBFL_WCSPLANE_8859_7) {
-			s = c & MBFL_WCSPLANE_MASK;
-		}
 	}
 
 	if (s >= 0) {

ext/mbstring/libmbfl/filters/mbfilter_iso8859_8.c

@@ -120,9 +120,6 @@ int mbfl_filt_conv_wchar_8859_8(int c, mbfl_convert_filter *filter)
 			}
 			n--;
 		}
-		if (s <= 0 && (c & ~MBFL_WCSPLANE_MASK) == MBFL_WCSPLANE_8859_8) {
-			s = c & MBFL_WCSPLANE_MASK;
-		}
 	}
 
 	if (s >= 0) {