Make sure utf8codepoint() advances the pointer even on bogus input.

This patch was from Francois Gouget, who found this bug in MojoSetup's copy
 of this function.

src/physfs_unicode.c

@@ -61,12 +61,13 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
 
     else if (octet < 224)  /* two octets */
     {
+        (*_str)++;  /* advance at least one byte in case of an error */
         octet -= (128+64);
         octet2 = (PHYSFS_uint32) ((PHYSFS_uint8) *(++str));
         if ((octet2 & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;
 
-        *_str += 2;  /* skip to next possible start of codepoint. */
+        *_str += 1;  /* skip to next possible start of codepoint. */
         retval = ((octet << 6) | (octet2 - 128));
         if ((retval >= 0x80) && (retval <= 0x7FF))
             return retval;
@@ -74,6 +75,7 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
 
     else if (octet < 240)  /* three octets */
     {
+        (*_str)++;  /* advance at least one byte in case of an error */
         octet -= (128+64+32);
         octet2 = (PHYSFS_uint32) ((PHYSFS_uint8) *(++str));
         if ((octet2 & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
@@ -83,7 +85,7 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
         if ((octet3 & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;
 
-        *_str += 3;  /* skip to next possible start of codepoint. */
+        *_str += 2;  /* skip to next possible start of codepoint. */
         retval = ( ((octet << 12)) | ((octet2-128) << 6) | ((octet3-128)) );
 
         /* There are seven "UTF-16 surrogates" that are illegal in UTF-8. */
@@ -106,6 +108,7 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
 
     else if (octet < 248)  /* four octets */
     {
+        (*_str)++;  /* advance at least one byte in case of an error */
         octet -= (128+64+32+16);
         octet2 = (PHYSFS_uint32) ((PHYSFS_uint8) *(++str));
         if ((octet2 & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
@@ -119,7 +122,7 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
         if ((octet4 & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;
 
-        *_str += 4;  /* skip to next possible start of codepoint. */
+        *_str += 3;  /* skip to next possible start of codepoint. */
         retval = ( ((octet << 18)) | ((octet2 - 128) << 12) |
                    ((octet3 - 128) << 6) | ((octet4 - 128)) );
         if ((retval >= 0x10000) && (retval <= 0x10FFFF))
@@ -134,6 +137,7 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
 
     else if (octet < 252)  /* five octets */
     {
+        (*_str)++;  /* advance at least one byte in case of an error */
         octet = (PHYSFS_uint32) ((PHYSFS_uint8) *(++str));
         if ((octet & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;
@@ -150,12 +154,13 @@ static PHYSFS_uint32 utf8codepoint(const char **_str)
         if ((octet & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;
 
-        *_str += 5;  /* skip to next possible start of codepoint. */
+        *_str += 4;  /* skip to next possible start of codepoint. */
         return UNICODE_BOGUS_CHAR_VALUE;
     } /* else if */
 
     else  /* six octets */
     {
+        (*_str)++;  /* advance at least one byte in case of an error */
         octet = (PHYSFS_uint32) ((PHYSFS_uint8) *(++str));
         if ((octet & (128+64)) != 128)  /* Format isn't 10xxxxxx? */
             return UNICODE_BOGUS_CHAR_VALUE;