Fix an unexpected error during tab completion for input like a.b. when a is not a record

[fingolfin]

Resolves #5241 (cc @zickgraf)

[FriedrichRober]

So I checked your branch on my laptop right now, it doesn't throw an error anymore.

Just want to make sure, that the following behaviour is what you wanted to achieve.
I run the following code on my laptop (MacBook Pro, macOS)

gap> G := SymmetricGroup( 3 );;
gap> G.test.

Pressing tab yields a list of all available commands:

*
+
-
.
.:=
/
16Bits_AssocWord
16Bits_CombiCollector
16Bits_DepthOfPcElement
16Bits_Equal
16Bits_ExponentOfPcElement
16Bits_ExponentSums1
16Bits_ExponentSums3
16Bits_ExponentSyllable
16Bits_ExponentsOfPcElement
16Bits_ExtRepOfObj
16Bits_GeneratorSyllable
16Bits_HeadByNumber
16Bits_LeadingExponentOfPcElement
16Bits_LengthWord
16Bits_Less
16Bits_ObjByVector
16Bits_Power
16Bits_Product
16Bits_Quotient
16Bits_SingleCollector
32Bits_AssocWord
32Bits_CombiCollector
32Bits_DepthOfPcElement
32Bits_Equal
32Bits_ExponentOfPcElement
32Bits_ExponentSums1
32Bits_ExponentSums3
32Bits_ExponentSyllable
32Bits_ExponentsOfPcElement
32Bits_ExtRepOfObj
32Bits_GeneratorSyllable
32Bits_HeadByNumber
32Bits_LeadingExponentOfPcElement
32Bits_LengthWord
32Bits_Less
32Bits_ObjByVector
32Bits_Power
32Bits_Product
32Bits_Quotient
32Bits_SingleCollector
8Bits_AssocWord
8Bits_CombiCollector
8Bits_DepthOfPcElement
--More--

[fingolfin]

@FriedrichRober thank you for the testing and feedback! Yes, this is intended behavior for now -- it just matches how GAP behaved in similar cases for years: when GAP has no matches to offer, it just suggest any identifier used as a record entry name. Of course that's not very helpful if you start with just say a.b. and it prints thousands of potential completions; but if you start with a.limitw and then it autocompletes to a.limitworkspace that could be helpful. That said, if somebody has a better idea how to deal with this situation, I'd be open to a PR to change it -- but this is way beyond the scope of this PR.

[fingolfin]

Backported to stable-4.12 via commit dbce444