patch 9.0.0105: illegal memory access when pattern starts with illegal byte

Problem:    Illegal memory access when pattern starts with illegal byte.
Solution:   Do not match a character with an illegal byte.

src/testdir/test_regexp_utf8.vim

@@ -1,5 +1,7 @@
 " Tests for regexp in utf8 encoding
 
+source shared.vim
+
 func s:equivalence_test()
   let str = "AÀÁÂÃÄÅĀĂĄǍǞǠǺȂȦȺḀẠẢẤẦẨẪẬẮẰẲẴẶ BƁɃḂḄḆ CÇĆĈĊČƇȻḈꞒ DĎĐƊḊḌḎḐḒ EÈÉÊËĒĔĖĘĚȄȆȨɆḔḖḘḚḜẸẺẼẾỀỂỄỆ FƑḞꞘ GĜĞĠĢƓǤǦǴḠꞠ HĤĦȞḢḤḦḨḪⱧ IÌÍÎÏĨĪĬĮİƗǏȈȊḬḮỈỊ JĴɈ KĶƘǨḰḲḴⱩꝀ LĹĻĽĿŁȽḶḸḺḼⱠ MḾṀṂ NÑŃŅŇǸṄṆṈṊꞤ OÒÓÔÕÖØŌŎŐƟƠǑǪǬǾȌȎȪȬȮȰṌṎṐṒỌỎỐỒỔỖỘỚỜỞỠỢ PƤṔṖⱣ QɊ RŔŖŘȐȒɌṘṚṜṞⱤꞦ SŚŜŞŠȘṠṢṤṦṨⱾꞨ TŢŤŦƬƮȚȾṪṬṮṰ UÙÚÛÜŨŪŬŮŰƯǕǙǛǓǗȔȖɄṲṴṶṸṺỤỦỨỪỬỮỰ  VƲṼṾ WŴẀẂẄẆẈ XẊẌ YÝŶŸƳȲɎẎỲỴỶỸ ZŹŻŽƵẐẒẔⱫ aàáâãäåāăąǎǟǡǻȃȧᶏḁẚạảấầẩẫậắằẳẵặⱥ bƀɓᵬᶀḃḅḇ cçćĉċčƈȼḉꞓꞔ dďđɗᵭᶁᶑḋḍḏḑḓ eèéêëēĕėęěȅȇȩɇᶒḕḗḙḛḝẹẻẽếềểễệ fƒᵮᶂḟꞙ gĝğġģǥǧǵɠᶃḡꞡ hĥħȟḣḥḧḩḫẖⱨꞕ iìíîïĩīĭįǐȉȋɨᶖḭḯỉị jĵǰɉ kķƙǩᶄḱḳḵⱪꝁ lĺļľŀłƚḷḹḻḽⱡ mᵯḿṁṃ nñńņňʼnǹᵰᶇṅṇṉṋꞥ oòóôõöøōŏőơǒǫǭǿȍȏȫȭȯȱɵṍṏṑṓọỏốồổỗộớờởỡợ pƥᵱᵽᶈṕṗ qɋʠ rŕŗřȑȓɍɽᵲᵳᶉṛṝṟꞧ sśŝşšșȿᵴᶊṡṣṥṧṩꞩ tţťŧƫƭțʈᵵṫṭṯṱẗⱦ uùúûüũūŭůűųǚǖưǔǘǜȕȗʉᵾᶙṳṵṷṹṻụủứừửữự vʋᶌṽṿ wŵẁẃẅẇẉẘ xẋẍ yýÿŷƴȳɏẏẙỳỵỷỹ zźżžƶᵶᶎẑẓẕⱬ"
   let groups = split(str)
@@ -560,6 +562,19 @@ func Test_match_invalid_byte()
   call delete('Xinvalid')
 endfunc
 
+func Test_match_illegal_byte()
+  let lines =<< trim END
+      silent! buffer ÿ\c
+      next ÿ
+      0scriptnames
+      source
+  END
+  call writefile(lines, 'Xregexp')
+  call system(GetVimCommand() .. ' -X -Z -e -s -S Xregexp -c qa!')
+
+  call delete('Xregexp')
+endfunc
+
 func Test_match_too_complicated()
   set regexpengine=1
   exe "noswapfile vsplit \xeb\xdb\x99"